"Steam doesn’t allow publishers to sell PC games and game keys for less money elsewhere"

So how come I can get cdkeys for some of they same games from online discount shops?

https://theverge.com/2021/5/...

Maybe cheap keys bought when game was on discount? Or maybe even if they sell cheap keys they are cutting it from profits so steam always receive same amount of cash for every single key sold?

Because those are grey area shops which often resell keys from another region or from when it was on offer / in a bundle. They don't particularly care about the terms of Steam, as unlike the developers they don't sell on Steam. Sometimes they keys are also be acquired through fraud.

These shops are bundling them up when there are Steam sales

Depends on the site. Sometimes it's a reseller and they just don't care about Steam's ToS. Sometimes they worked out a deal with Steam in advance (Humble Bundle used to have some kind of deal with Steam to allow them to offer discounted games, but I don't think that still applies today).

And sometimes the keys are illegally obtained.

I've had too many bad experiences with resellers to trust any of them at this point. Which is part of the reason why I'm on the fence about Steam now. On one hand, they're definitely engaging in anti-competitive practices. On the other hand, Valve has never defrauded me or refused to honor their refund policy in the 16 years I've used Steam. That trust is hard to build up.