In fairness to him, he offers the posters an opportunity to propose a technical solution and responds to all the posts that do it. It is interesting that nobody in the thread went to check in the code of mpv, smplayer, etc. to see how it's done there. Surely this would be the best response to his request for technical suggestions?
Maybe because these users just can't code?
But abstractly, it's absolutely possible to write a program that decodes frame at a time and displays them slowly.
Now, whether there is some architectural difficulty based on design decisions within their player, or any player, I don't know.
Edit: I guess downvoters have never worked with a video decoder api before? I just read the link and it seems like the rationale is to not seek one frame backwards because you'd have to seek to a key frame and waste some work? That's not the same as it not being possible.