And it’s “apseudestatos”, not the other. So it’s not entirely wrong, and I apologize to your source.
“Apseudestatos” is a poetic sort of word. Using this word, the poet Archilochus described Zeus as “the most unlying foreteller”, meaning, the most absolutely truthful foreteller, because “he himself holds the outcome.” (You can’t be wrong if you not just know the end, but make it happen with your power. Zeus was the king of the gods and made stuff happen, which was much more than just being an oracle god. And yes, Zeus’ oak tree oracle at Dodona was very trusted in the ancient world.)
So it seems that most Greek dictionaries define this word as “the most truthful possible”, “absolutely honest”, and so on.
Anyway, the meaning of Amphilochius’ remark is that:
A) he needed a nice highfaluting three dollar word to fill out his next to last line, so he could end the poem in a resounding way.
B) he was assuring Seleucus that his list was absolutely true and correct, so far as he knew; he himself wasn’t lying.
He admits that some of the names on his list are still being debated (like Esther and Revelation), and it probably wouldn’t have surprised him to hear of other valid ones. But he’s not claiming that it’s a full or unerring list; he’s just claiming that it’s a truthful list.