The replay system doesn't just record the inputs, it records the inputs of the characters, so it's essentially playing the game again every time. The only way to make a game like this have an mp4 conversion is if Ivysly incorporated his own screen-capturing software which would be REDICULOUS.
I have OBS. I have been able to effortlessly record the 14s clips of my replays and save them
Viewing post in Replay conversion to mp4
Not everyone has a good enough computer for this.
There's no good way to have an in game way to convert stuff. At best someone could set up a website or smth where a bot takes replay files, inputs them into the game instance, and then spits it back out for you to download as an MP4. But that'd be an exorbitant amount of effort, and wouldn't be official. But I theorize it'd be the most reasonable option regardless.
Idk how that works in godot but with other engines, there are libraries or extensions that can handle that or if you really want to go the "painful but classic way", just play a frame, capture it as a screenshot then go onto the next frame until the game ends then make a video out out it. Idk how audio would get taken into account with this method tho. honnestly if the game was made in Unity (that's the only thing i have experience modding) i would have liked to make a mod that exports replays in video