If you're curious what happened and what was fixed... eh... The exported game turned all the media file references to lowercase, and a lot of videos had uppercase characters. It's not a problem in Windows, but Unix systems like to have more characters at their disposal and chaos was born. Every file with uppercase characters was just ignored.
Just doing a shell command to rename everything to lowercase did not do anything because Steam also ignores uppercase, so it just told me they're all the same files and refused to rename any of them :(
Guess what I did for the last five days (besides sleeping and hiking and drinking too much tea and lazying about and hating myself). Yes, I renamed hundreds of files, both in the code and the directory. I made them all lowercase and also added a character at the end of each offending file, so Steam would finally make the extreme effort of changing those file names to work. On the upside, I finally had a chance to go through our chaotic naming convention and make it even worse.
Turns out, this problem is plaguing Steam since 2013. and some pretty big games also had chaos with their Linux distribution. Unlike them, we don't have badly paid slaves to do the hard work, we only have badly paid ourselves, but here it is, finally. I tested it out in most encounters, but who knows if I did EVERYTHING correctly.
Both Windows and Linux users, please, if you see a black screen where a video should be, please let me know, it means I messed something up. My "QA team" still consists of whoever publishes their gameplay videos on youtube and twitch D:
I would also like to thank Linux users who went to game discussion to report the problem, find a quick fix for more impatient users and help me out with this embarrassment <3