That saying too often gets used as an excuse to not even try moving away from patronizing a harmful business, as though it isn’t worth any inconvenience since we’re screwed no matter what.

Isn’t that because Dan hasn’t open-sourced his project yet

Wrong: https://github.com/pixelfed/pixelfed/blob/dev/LICENSE

and doesn’t let anybody else contribute?

Hmm, while there have been 158 contributors, a very small percent were from the other developers: https://github.com/pixelfed/pixelfed/graphs/contributors

This information is pretty easy to look up.

Which comment in the issue thread leads you to believe that?

The developer’s closing comment is that it wouldn’t be worth it to implement that feature in Lemmy.

One way would be by implementing features the Lemmy devs have no interest in such as better interoperability with other fediverse platforms. If any added feature turns out to be well received and in demand, it would pressure the others to implement similar.