Headline is a little misleading. There is a One day waiting period when you enable installing third party applications. After that you can install them indefinitely. It’s to stop active scams. I agree it’s BS, but it’s a lot less BS than I was expecting and what the headline/comments are making it out to be.

Okay but what’s stopping the scammer from telling you to install his app from the play store which is less trustworthy than, say, f-droid?

They would never admit to the play store malware problem.