Specific beats general if they directly contradict, but these don't in any way. Having a new potential source of inspiration (other than the DM's choice) doesn't change anything; just like DM-granted inspiration, if the target creature already has inspiration then nothing happens. A DM can't grant multiple banked uses of inspiration, so why should this NPC be able to?
When the base rules say "Your DM can choose to give you inspiration ...", they don't say that the DM must be the only possible source of it. So that's also not a conflict between base rules and this module. New ways for something to happen aren't conflicts, they just slot into the existing rules framework.
One specific rule about something doesn't mean that all other rules stop applying, only ones that actually contradict. (If that was the case, "inspiration" wouldn't even have any mechanical meaning because the whole game concept of what inspiration is is defined elsewhere as well.) Nothing about any of the wording in anything you quoted gives any hint that it should be able to stack, therefore there's no conflict with the general rule.
But because "grants inspiration" and "has it or doesn't" are not directly contradictory, then there is no conflict there to resolve?
Correct. "Grant" means the target creature now has inspiration. (Whether or not they had inspiration to start with.) If they already had inspiration, it's a waste of your action, but nothing in D&D stops you from wasting (part of) your turn. The known fact that inspiration can't stack means we need to interpret this phrasing as "set inspiration status to "yes"".
(@daveman's example of Heroism and non-stacking temp HP is good.) e.g. "The NPC grants you some water" - "thanks, but my canteen is already full".
A sensible and obvious interpretation exists that's compatible with both rules. Therefore it's clear that's how the specific rule works. General rules and definitions of terms are the framework that specific rules are built on, and guide their interpretation. There's only a true conflict if you can't find a clear sensible interpretation of the wording that's compatible with the general rule. (Usually because the specific rule is pretty explicit about it.) e.g. the general rules around spellcasting tell you how to interpret rules about specific things that let you "cast" spells.
If a creature had an ability that was intended to override the general rule, it would tell you more explicitly. For example, "As an action, [NPC] can inspire another creature, giving them advantage on a d20 roll of their choice. A creature can have at most 5 uses of inspiration saved up, but disappear on a long rest." i.e. define a way for them to stack. There's no such language, and therefore no reason to expect that the general rule stops applying.
There are cases where two rules contradict and it's not clear which is more specific or general, so the DM has to make a ruling for how they interact, but this is not one of them.
It's the same kind of question as "you can do X as a bonus action" doesn't override the fact that you can only do one bonus action per turn. I think everyone understands that's how it works even though class feature descriptions of ways to spend your bonus action don't bother to repeat that fact. Or that something stopping you from taking Actions also stops you from taking Bonus Actions.
Or a feature that lets you make an Attack some way other than taking the Attack action on your turn. It doesn't replace that mechanic.
(Correction: this NPC can only do this while Strahd is in sight. But just as an example, consider an NPC that really could do it every round without limit, even when there's no danger or threat present. The same applies during a social encounter with Strahd for the actual NPC; it would be super broken if you could bank up stacking inspirations while chatting.)
A creature that can give out inspiration every round means you should expect the whole party to start every combat with inspiration (unless they spent it on an ability check that started combat), and for every out-of-combat ability check to be made with inspiration. (Unless two whole-party checks or saves in a row used up inspirations before the NPC had a chance to re-inspire each PCs, one per 6 seconds.)
If it could stack without limit, this would obviously be unbalanced. Given 10 minutes outside of combat, the NPC could hand out 100 total inspirations spread across the party, or 600 per hour of being awake. So you'd expect the party to roll every check, save, and attack with advantage for the entire rest of the time they accompany this NPC, and probably for days after parting company if it doesn't expire on a long rest. That's a massive boost, and another clear sign that this interpretation is not intended.
(Sanity checks like this are more helpful in deciding on a ruling for your game, even if there is a loophole in the RAW. Especially for modules or UA material, loopholes are occasionally possible if RAW doesn't actually say what the designer intended.)
This is very similar to a bard giving out Bardic Inspiration: a creature can have at most one die from the bard. Bards have a very limited supply, but can inspire a creature of their choice each round for a few rounds.
Bardic Inspiration only lasts 10 minutes, so re-inspiring a creature could refresh the duration, but otherwise be a wasted action. Nothing in the rules stops you from wasting your action by trying to inspire someone who already has inspiration, or taking the Dodge action twice on one turn (as a monk with action + bonus), or re-casting a spell that was already active.