I'm not exactly sure if you wanted an explanation answer for that probably rhetorical question, but /dev/dri/card0 is my gpu.
I tried the same thing with just one screen, but it didn't help, as expected.
I guess you might be interested into my package list for packages like Firejail (Don't worry, I'm trying to keep my system as clean as possible, so the list shouldn't be too big): https://termbin.com/pmlt