The only way I know to harden Linux Mint is using the Debian edition. Using LMDE, you can (unofficial) use Kicksecure to harden the base system. This isnt a great solution since the Linux Mint software is untested with Kicksecure and may/will reduce the security of the overall hardening.
I watched the video. Yes, if your sandbox config is weak then it will allow sandbox escapes. I agree the should default should be a secure sandbox. Bubblewrap offers the opportunity to shoot yourself in the foot. Look into the others tools I mentioned if you want to see different implementations. Sydbox is the one I think is the most interesting.