I’ve never seen this particular accusation anywhere, do you have examples? You’re worried that someone will hack a math textbook to teach incorrect math that’s not obviously wrong or obviously hacked, but misleading and damaging to math education somehow? Or what kind of threat exactly are you imagining? Who’s got a secret plan to corrupt math education, and how does that work when math involves exercises and proofs? Brandolini’s Law certainly finds application in politics, but it’s news to me if anyone has ever taken the time to intentionally develop fake but believable academic curricula. Why would someone go to that level of effort, what would be the political agenda, and what makes you think it would get very far without getting caught and taken down?
I don't necessarily think that math is where this critique is the strongest. As you say, this happens most in political arguments, and this critique is strongest when talking about history books. I listened to a rant about this once and I think it is adjacent enough to be relevant as a critique here:
While I attended a church in a different state, the Sunday school teacher decided to get on a talking point aka rant about how history books are going to be changed via an opaque technology process to save kids backs from heavy textbooks and this was the BIG BAD. The entire topic kinda blindsided me at the time as I was leaning toward khan academy and other progressive teaching techniques at the time. (Still do, but i have had a number of bad experiences with subscription models for knowledge that is a prerequisite for certain jobs).
I bet that math textbooks are the more exact science, so it will probably pass over well, but some people (conservatives who don't trust change) will probably mount resistance if not addressed.
I just wanted to say that there are two sides (monetary incentives) to the question "should we adopt", and it directly shapes the narrative (and may cause a narrative to change into a political one).
Regarding "never heard of deliberately fake curriculum", i agree that i havent seen a deliberately fake math curriculum, but i have seen a number of "montressouri schools" that have no credentials and have not updated based on science for years. Most "charter schools" have a similar standard.