I think the point is that even if an extension comes from a trusted source, the developer could fairly easily push out an update that turns the extension into malware. Check the GitHub link in another comment below where the developer posts the solicitation emails he gets on a regular basis offering to monetize his extension. He isn’t selling out, but maybe not every dev is as willing as he is to forgo a potentially lucrative offer.
And there are cases where this has already happened: https://www.bleepingcomputer.com/news/security/-particle-chrome-extension-sold-to-new-dev-who-immediately-turns-it-into-adware/ There are probably more recent cases too, but this was the first one I could find.