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.

To be specific: from trusted developers. Installing them only from the official repository (is it still possible to reasonably install them any other way?) won't help if a dev sells such an addon. On the other hand I cannot imagine someone like Raymond Hill (the uBlock Origin dev) doing it, considering his track record.