Can a Warlock with the Book of Ancient Secrets invocation add unintelligible spell scrolls to their book?

Question

Can a warlock with the Book of Ancient Secrets eldritch invocation copy spells from a spell scroll from another class' spell list? For example: could a level three warlock copy a spell scroll of detect magic (a 1st-level spell)?

My confusion arises because the spell scroll rule (DMG p. 200) states:

If the spell is on your class’s spell list, you can read the scroll and cast its spell without providing any material Components. Otherwise, the scroll is unintelligible.

And the warlock rule for the Book of Ancient Secrets eldritch invocation (PHB p. 110) states:

On your adventures, you can add other ritual spells to your Book of Shadows. When you find such a spell, you can add it to the book if the spell's level is equal to or less than half your warlock level (rounded up) and if you can spare the time to transcribe the spell. For each level of the spell, the transcription process takes 2 hours and costs 50 gp for the rare inks needed to inscribe it.

Can a warlock transcribe an unintelligible spell, or are such spells intelligible to warlocks with the invocation?

Answer 1

This is a case where specific beats general. Book of Ancient Secrets does exactly what it says, which is that it allows you to:

[A]dd other ritual Spells to your Book of Shadows. When you find such a spell, you can add it to the book if the spell’s level is equal to or less than half your warlock level (rounded up)

3/2 = 1.5 ~= 2. You can transcribe any and all rituals your warlock finds that are <= spell level 2. Detect Magic is level 1 and a ritual. 1 < 2, so you can transcribe it.

Answer 2

Yes, you could transcribe it. But...

As J. A. Streich pointed out, your Warlock would be totally capable of transcribing a scroll for a ritual spell, even if that spell is not on their class's spell list. This is one of the things that makes Warlock Ritual Casting particularly excellent. However, nothing in the Book of Ancient Secrets feature stops the scroll from being "unintelligible" if you simply read it (silently, without attempting to cast it).

Thus, if you were to find a scroll for a Ritual spell which isn't on the Warlock spell list, you wouldn't know what spell it was until you had finished attempting to transcribe it, and could legibly read it from your book. In fact, if you just found a spell scroll, you might not even know whether or not it is a ritual spell and eligible for transcription.

This is potentially a serious risk, because when you attempt to copy a spell from a scroll, regardless of whether or not you are successful,

the spell scroll is destroyed. (DMG, p. 201)

As such, before you attempt to copy a scroll into your book, you'd better have someone who does have that spell on their class list confirm what spell it is, and whether it is a ritual spell.