I still don't get this. As far as I can find, Steam doesn't allow steam keys to be sold cheaper elsewhere, but they don't bother with prices of games in other stores.

This is tricky. Officially Valve doesn't have any rules about non-Steam game prices on other stores. Unofficially evidence has been put forward by way of emails between developers and Valve that seem to show that Valve unofficially requires price parity with other stores and will punish games that offer lower prices elsewhere.

The charitable interpretation is that their policies are worded confusingly and some of their agents are misinterpreting the rule requiring Steam key prices to be uniform as applying to non-Steam keys. The uncharitable interpretation is that Valve knows such a policy would get them in hot water with anti-monopoly laws and so they're careful to make sure it stays an unofficial policy.

That's... Weird, given I usually get games on sale from Humble if I'm getting them on steam. Might just be full price?

Steam, and Epic, both have exclusives. Steam is more incidental (some devs just don't bother releasing elsewhere), while Epic had a deal going on for devs that released exclusively on Epic for the first 6 months of the game's life. Don't remember what the deal was, but it was a marketing thing to try and get people over to Epic. After the 6 months was up most devs also released to Steam.

Is that true? I've seen devs list their games off steam for steamprice -30%. Often you can go to a devs website and buy the game $10 cheaper. However I still buy it via steam because i feel like i'll forget about the game if its just a key in my email.

If I'm remembering correctly, price parity only applies to keys sold off platform, rather than the game itself because they don't profit from the keys being generated.