This sounds lile terrible advice. Not only does it take a long time to get familiar enough with a code base of reasonable complexity to make a contribution of any interest, but not all open source projects have the capacity to review and process external PRs.

You are essentially trying to steal time from people who already invest a lot for the community for your personal profit. This feels parasitic.

My advice to electricians starting out: find a construction job underway and fix wiring issues for free.

His advice might not be wrong, but there is obviously something wrong with software engineering as a profession.