a huge number of spelling mistakes screams spam to me

Do you mean to say, you’ve learned to associate spelling errors with spam because most of the spam you see… the spam that gets past your spam filters… has a lot of spelling errors?

The best way to deliver spam is to make it indistinguishable from legit messages.

That’s just not true. The best way to deliver spam is to send it from a reputable address, and to avoid looking like spam.

Bayesian filters need to be trained by a user identifying email as spam. From those emails it learns which words frequently appear in spam emails. Including spelling errors means more unique words rather than words that look like spam.