I'm pretty sure the answer is YES, it is a bluff even if you aren't 100% sure it's a lie, because you aren't 100% sure it's the truth either. So a bluff. That's my verdict.
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We had a similar argument in a table once (some in-character amnesia was involved). We agreed that bluff/diplomacy are based on what the character knows. If he/she has reason to believe it's a lie, then it's bluff, otherwise it's diplomacy.
Seems reasonable, but then maybe Truth Teller and similar flaws that rely on the character knowingly lying might have to not apply here.