Demos were really useful for helping validate competitive play too. While certain anti-cheat programs were available such as PunkBuster (Quake 3), having gaming ladders request everyone records a demo and upload it from their POV was a very low friction way to deter cheating. The idea being, no one looked at them unless there was suspicion so it wasn't even a time sink for administrators.

No fancy kernel level anti-cheats. Just ensure matches were played on legitimate servers and demos were recorded.

Also, back then live streaming while playing was usually too much of a computational and network burden (56k modems), but casting was just coming around as being a thing and certain Quake 3 mods had spectator modes that let someone streaming spectate you from the first person live which also helped deter cheating. There was even split screen spectating modes so you can follow the action (useful for 4v4 games, etc.).

Carmack and team really made something special back then. The ideas they had and what they did with their tech on relatively low end hardware was remarkable.