One problem, if it even is a problem, is that NaNoWriMo uses a honour system for the word counts. They had word count verification in past but it accepted "obfuscated" manuscripts (each letter replaced with random letters, or something similar). They don't have any way of assessing the quality of the writing, and that absolutely goes against the spirit of the event anyway.
(For a lot of writers this could be the first time they try writing a novel. Last thing they want is an algorithm rejecting their work if it sounds too much like AI. That'd be fucking horrible.)
Ultimately, NaNoWriMo isn't about quality of writing, it's about getting into the habit producing text for 30 days. Using any AI to create novel text goes straight up against that idea.
I've always said it's OK that you're not producing your 100% best prose in some NaNoWriMo days. Or just come up with tangentially related ramblings. It's, uh, a postmodern composition technique. But try to use a brain, OK? AI will just produce irrelevant nonsense. One of my fave technique is that if I'm really desperate in NaNoWriMo, I fire up lipsum.com and generate a day's worth of lorem lipsum nonsense. I can do it once. Then I must remove words from that block if I exceed the daily quota.