They do not allow steam keys (free to generate steam licenses of games) to be sold cheaper anywhere else for less than the game is sold for on steam.

That itself is false too with a quick lool at isthereanydeals showing lot of steam games being sold cheaper outside of the steam store.

Even the Steam key guidelines don’t explicitly state that steam keys can’t be sold cheaper.

It’s OK to run a discount for Steam Keys on different stores at different times as long as you plan to give a comparable offer to Steam customers within a reasonable amount of time.

partner.steamgames.com/doc/features/keys

Key word being comparable which si why if you are a user of isthereanydeals or /r/gamedeals you’ve likely gotten most of your steam games from outside the official Steam store.

I think some people just assume Steam sales must be the cheapest and don’t look beyond it.