Hilarious - Are they right?How would one find out? If as OP stated, their skills lined up and matched the job requirements, how will they ever know?
Startup recruiting seems a lot like modern dating: a lot of terribly communicated expectations resulting one party effectively "ghosting" on the other, ending in a lot of frustration and apathy from one party at the other because of said terrible communication-and that resentment or 'baggage' being unknowingly carried with them to the next interview cycle.
"Not a good fit" can mean anything from asking too much salary, to being just a month short of the necessary 5 years of experience for a tech stack that's been around for 3, to not wearing the right color shirt to the interview. You'll never know what "not a good fit" means because it seems like once a company decides you're not the perfect unicorn, rockstar candidate with a pocket full of ninja stars, you never hear from them again.
But don't worry, your resume is kept on file for other roles.