Is this video a legitimate way to get Linux on LineageOS via Termux or is there a better recent method?

Depends what you mean by "Linux" here.

It's probably not the kernel itself, so do you mean

• a terminal e.g. a working shell where you can run commands e.g. ls | wc -l ?
• headless containers, e.g. services like Immich accessed elsewhere?
• a window manager e.g. KDE or Gnome?
• a software with a visual interface, or GUI, e.g. GCompris?

Based on that then one can answer if Termux is sufficient (or "legitimate") or if something else is needed.

PS: You can read some of my notes on termux on different Android devices at https://fabien.benetou.fr/Tools/Android