This is fundamentally not true.

If your writing is weak, traditional publishing will show you that. Self pubbing won't. There are a tremendous number of deluded self pub authors. Nobody in trad pub is deluded, because deluded writers don't get there.

There's no reason to improve your writing with self pub. In fact, even if you're very good, if you self pub, you're likely not to reach enough people for it to matter. You have no marketing budget behind you. Nobody is putting you on the shelves. There are probably some great self pub authors out there that nobody will ever read, just because nobody will ever hear about them. But I'm guessing - because there are over 50,000 new self pub authors every year, and they publish with zero quality check, and zero marketing.

There's an element of luck in each. But in terms of competing, self pub is the opposite of that. It says to people, "Hey, publish whatever you want. You'll never get read anyway." Because that's the overwhelming truth for the vast majority of self pub authors.

Anyone can improve their craft over time. But rejection is a powerful thing in encouraging someone to grow. There's a real fear right now in trad pub that people who could become good authors will never become the authors that they should have been - because they'll self pub, see their book sink and disappear into the quagmire of garbage, and instead of going back to the grindstone will feel that they've published; they've done that. When in fact they've done nothing but upload a file to Amazon.