I beleive that compression is done client side in the app.
To get a similar result you'd need to compress it in the app before sending the data to the server.
I don't know if there's any native browser tech that can do that?
So it's unlikely to be able to get that to work in a mobile or desktop browser.
I'm guessing the only way to do it all client side is you'd need your users to be using a native forum app that does the heavy lifting video conversion on the phone before uploading it to the post.
The other option is to use ffmpeg server side to convert it.