The next email asks "How many years of experience do you have working with Flutter only?". Doesn’t the question show that the founder knows absolutely nothing about software development. I am furious at this question. Am I over-reacting?
I find this metric to be severely outdated and not reflective of software development skills at all for reasons such as the non-linear nature of skill acquisition and the rate at which technology evolves. Especially for a newer framework like Flutter.
Asking how many years you've actually spent with a framework is a valid question, and you're going to get it again. So look inward. Do you really have 6 years of actual experience shipping apps with it? Or is it 2 or 3? Maybe one? Have you shipped at all?
And if you have shipped a Flutter app, pivot the conversation to that instead. Point out what you did. And then it doesn't matter how many years you did (or didn't) spend on it because you're giving the the interviewer what he is looking for. What's that Spolsky tag line? Smart and Gets Things Done(tm)
Apply, hope for the best, and move on if it doesn't work out.