Yes. Disintegrate is not enough to permanently destroy a demon
Specific Beats General
The most direct and complete treatment of this question is probably this line from the Eternal Evil entry in the MM:
The only way to truly destroy a demon is to seek it in the Abyss and kill it there.
That's very specific and explicit. Disintegrating a demon anywhere other than the Abyss is not the "only way to truly destroy" it, and so doing so won't destroy it.
We can also look at specific-beats-general outside of that line. In this case, I submit that the order of specificity goes like this, ordered from more general to more specific:
- Creatures can die and be resurrected
- Disintegrate inflicts a mode of dying which limits which generic resurrection methods will work and is applicable to creatures,
generally
- Demons (and various other immortal-ish creatures) have a specific
feature which overrules the mechanisms of generic creature death
This isn't an airtight ordering of specificity, but my case is that Disintegrate describes what happens to creatures in general, while demons are a specific type of creature with a trait that makes them different from generic creatures. So Disintegrate can happen to everyone, but not everyone is a demon, and so the demon's specific rules surrounding their death always supersede what Disintegrate might do to any random creature.
Regarding reading through the description of the spell and demonic immortality, I see where the confusion lies, and I think that it is reasonable to make the opposite argument. I see two assumptions in the question which suggest the opposite ruling, so let's look at each of those in turn:
1. The specific rule of Disintegrate
A disintegrated creature and everything it is wearing and carrying, except Magic Items, are reduced to a pile of fine gray dust. The creature can be restored to life only by means of a True Resurrection or a wish spell.
It's the bolded portion that seems most relevant to the question. My reading of it is that it is meant mainly to rule out the use of Resurrection, a weaker life-restoring spell. True Resurrection includes a key line which Resurrection does not, and which is meaningful here:
The spell can even provide a new body if the original no longer exists, in which case you must speak the creature's name.
This line interacts with Disintegrate because, deductively if nothing else, a pile of dust is not a dead creature's body. This matters a lot to beings that have what we might call "single use" bodies-- once the body is done, so is the being. But there are other categories of creature for whom the body is more like a hat than a central element of their being. Consider creatures with the Rejuvenation trait in their stat blocks for comparison, like Nagas and Liches:
Naga
Rejuvenation. If it dies, the naga returns to life in 1d6 days and regains all its hit points. Only a wish spell can prevent this trait from functioning.
Lich
Rejuvenation. If it has a phylactery, a destroyed lich gains a new body in 1d10 days, regaining all its hit points and becoming active again. The new body appears within 5 feet of the phylactery.
These are (or can operate as), essentially, "new body" effects; the condition of the original body is irrelevant to rejuvenations, and no portion of the original body is needed at all.
This is similar to the "dissolve into ichor" aspect of a demon dying on a non-Abyssal plane. Even when Disintegrate is not involved, a slain demon does not return to its previous body: the foul ichor remains where it is, and the demon re-forms in the Abyss.
The obvious counter-argument is that demons do not have any immortal nature described in their stat blocks, while Nagas and Liches do, but I think to suggest that the entry in the MM saying that demonic-style immortality applies to all demons actually applies to none is a facile argument. Nonetheless, it remains a valid argument whether I find it convincing or not.
2. A fiend dissolving into foul ichor does not seem to be a step in the process of a demon persisting after death
[...] the fiend dissolves into foul ichor. It then instantly reforms in the Abyss, its mind and essence intact.
The wording could be clearer, and so the quote can be interpreted in two ways: the fiend dissolving into foul ichor is a necessary step which must occur before the re-formation in the Abyss, or the two events are unrelated (so, a fiend slain in the Abyss might still dissolve into ichor there, and a fiend killed elsewhere might reform in the Abyss whether or not the dissolution occurs).
I suggest that the latter interpretation is better grounded than the former. Outside of one particular interpretation of that line in the Eternal Evil description of demons in the MM, there are no rules, mechanisms, or narrative descriptions I'm aware of which indicate the fiend dissolving is necessary to the process of the demon re-forming.
That, in turn, suggests that such an interpretation requires us to read new details into the rules which are not there. Relying on the questionable reading of the line itself as evidence for that reading of the line is tautological. This doesn't mean that that reading of the line is definitely wrong, only that it is not more supported than the other reading, and so there is no reason to favor it.
tl;dr:
- Demonic immortality does not depend on preservation of a demon's
original body, and so it should not matter if that original body
exists as ichor, dust, or anything at all
- Analogous methods of immortality involve new bodies rather than
rejuvenation of old, dead bodies, and so there is precedent for a
similar effect for demons
- Demons, as a specific type of creature with specific attributes,
should see those attributes supersede a spell's generic effect on
creatures, generally.
- There is a method of permanently killing a demon which is explicitly described, very specific, and is exclusive of all other approaches, and using Disintegrate is
not that method.
