These kinds of confident assertions about "facts" and "narrative" are also a kind of narrative of their own; they are designed to fit into and reinforce a broad narrative about the world any social outrage is usually just the result of poor research or snap-takes rather than actual differences in ideologies or opinions, and that people's default reaction to them should be to assume everyone is just overreacting because they always do. So let's step away from that narrative as well, and instead look at something that's a bit more tangible.
For anyone who's curious about the actual reasoning, the minutes of the meeting are online (https://core-docs.s3.amazonaws.com/documents/asset/uploaded_...). So you don't need to take a secondhand opinion, you can read the parents' reasoning for yourself. That's a good factual place to start.
And from the minutes, it is very hard to conclude that the objection to Maus was that Maus isn't a textbook. The overwhelming objection was concern over several swear words and a naked image, all of them censored in the books that were to be used for the curriculum, and an objection to minor depictions of nakedness (of cartoon rats being literally marched into camps), and an objection that bringing up disturbing topics like hangings, child deaths, or the existence of swearing/sex to 8th graders was inappropriate in a conversation about the holocaust. Criticism of the board's decision isn't based on a misunderstanding of the facts or people jumping to conclusions without reading the minutes, but rather an assertion that teaching kids about the holocaust necessarily requires mentioning the parts of it that were disturbing.
In fact, far from the removal of Maus from the curriculum being about whether or not Maus is a textbook, the minutes contain criticism from a parent of other school textbooks that contain swearwords. Which should make it really clear that the issue wasn't that Maus isn't a formal text. I do not know how to read the full minutes in a way that supports the parent comment's argument. It confidently misrepresents the entire conflict with no evidence, even as it claims that everyone else is doing the same.