The Massachusetts Positive Negative

I guess in MA, the phrase “So don’t I” means the same as “So do I”

How that even works, I don’t know, but Dan and I were arguing about grammar and negatives, so I looked it up, and it’s a real thing!  Here was the offending phrase, wrong to me, but I guess right to that crazy East coast :

i do, for the most part (or so i think), and so don’t most of the people i know personally that i follow.