One difference. On Itch.io you can't claim a free game, but you can claim a game with a 100% discount.
The game appears to be paid, but with a 100% discount, so in theory you should be able to claim the game. If you didn't receive the steam key, it might be because they ran out and in that case you should contact the developer directly. But searching on Steam, it says that the game is Not Available, so it gives the impression of being something abandoned by the developer.
Notice: The Great C is no longer available on the Steam store.
What caught my attention is that the page violates Itch.io's quality guidelines and yet it is indexed.
Pages that only contain a link to another platform do not represent the kind of content we want to promote, don't meet our quality guidelines, and may not be eligible for indexing in our discovery features.