I've heard this expressed several different ways, like creating your own job or learning by jumping right in, and it always completely ignores the value of professional experience.

Internships and first jobs are all about learning the realities of the work world and building up your professional network. If you're trying to build a B2B SaaS app, it really helps when you understand at least one of those "B"s instead of just guessing.

There will always be anecdotes of "I started my first business at 14 and have been my own boss ever since" but my (anecdotal) experience shows that most successful entrepreneurs are people who leverage their professional experience and network in order to get their first customers, co-founders, and so forth.