Yes, it's a missing library for video decoding IIRC.
For some insane reason, it started working around Proton 7 or maybe 8, but then broke again in 9. This is also true of the videos that play on monitors in Subnautica IIRC.

GE Proton has that fixed. I'm using GE Proton 10.4.

Steam makes installing GE Proton absurdly simple. Just extract the tarball of the latest version into ~/.steam/root/compatibilitytools.d/, then restart Steam and change your config to use the version you just extracted. Note that it'll be at the very bottom of the list, below all the stock, Steam provided versions.

Why would anyone actually *want* background video playing on a menu screen ? <smh>

I played it for years not knowing they were supposed to be there... I always thought the pale blue horizontal lines were an odd background choice. It wasn't until I switched to Proton 7 (or 8) when I saw them for the first time, which made me want to dig around to see why I hadn't seen 'em before.

On the other hand, if I came from Windows where they'd play by default, I'd probably notice them missing too and wonder what else is broken.

I ended up making a guide:
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=3515247467