Don’t all modern browsers allow you to disable auto-playing of video, even per-site if desired?

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.