The claim that you would have to decode all previous frames in the entire video is... completely baffling to see coming from the dev. He's arguing a stupid technicality that a video might not have keyframes. That's not a reason to omit the feature entirely.
Going back from frame 500000 to frame 499999 is in the limiting case as complex as seeking from 1 to 499999, and in most cases far better.
I think the forum thread would be better answered "you do it, I don't need this feature" which is basically the gist of it and is a completely fair answer.
Seriously, I don't get these people that have infinite demands from open source developers and contribute zero.
The commenters are trying to point this out, which misses the reality that the developer probably isn't going to budge from their requirement of universal support.
That dev's rationalization also sends a signal to any commenter with the technical chops to submit a PR, that it will probably be rejected for not supporting 100% of the codecs. I have no doubt people who could do it, over the years looked at that thread and concluded it would be a waste of their time.
It's not about "technical chops", it's about being constantly available, reliable person that shows to contribute day after day. If you don't do that why should a dev make the scope of their work bigger if they won't be able to keep putting the same quality of work?
They shouldn't...but that's not at all what the primary developer said.
The issue is the primary dev seems to require reverse seek functionality to work with all codecs, and since some obscure codecs can't efficiently support it, they're not interested. Their challenge to others to submit a PR is counteracted by all the signs that they might provide a high-to-impossible barrier to approval.
It's not clear that even a trusted contributor would be able to sway this person's mind. Most likely, contributors either agree, or keep quiet on the issue if they disagree.