PopOs fucked up a bit leaving that iso live for so long with the steam cache issue, but Linus has to take responsibility here to. It was not "totally on the OS" there where many off ramps that where missed.

For example:

• On the pop shop GUI if he first installed the system updates that would have run a background apt update avoiding the issue. There was a red bubble indicator you can see in the video that should have drawn his attention if he was not in a rush.
• On the pop shop steam page there was a list box that would have allowed him to install the flatpak version of steam, its not like it was hidden or anything it was right next to the install button and its was in fact bigger than the install button.
• Most people googling the terminal way of installing apps in linux would have also run apt update but he either skipped this bit or just ran the first thing he saw on google.... He probably would have also typed in "rm -rf /*" if google told him to without a second thought.
• He could have also just read what the OS was trying to tell him, pause, have a think and maybe ask some one like emily? But no he was in a rush so no time for that.

He got into this issue mainly because he was rushing, it was more entertaining and created drama.