A particular version of the steam.deb file got pushed to the Ubuntu repository that had some bugged prerequisites. If it didn't recognize the exact DE you were using, it would deem it incompatible and try to remove it. This wasn't a problem for a lot of more popular distros running more popular DEs, but it caught Pop!_OS and their goofy fork of Gnome.

Prior to That Video, this issue had been found, reported and fixed. But it just so happens that bugged version was what was in the apt cache at the time the install image Linus used was made.

Further, Pop!_OS didn't ever suggest Linus do a system update after a fresh install, nor did it update the apt cache when launching the Pop!_Shop, so that out of date apt cache was used.

Further, instead of googling an error message he was given and trying to solve the problem, the child that walks like a CEO threw a tantrum about "you have to use the terminal" looked up the command to install Linux via apt, also failed to do an update first (which you typically do; most guides about installing something tell you do an apt update && apt upgrade first) and he just plowed right through to a borked install.