It's not a "mistake" it was an intentional decision. Plenty of games are available on steam and itch.io, they didn't accidentally switch it to steam exclusively. Also, the contradictory statements of "um actually it's a completely different game " and "I wish I could honor your previous purchase of the game I made a mistake UwU" really irks me. Its either a completely different game and they shouldn't have felt the need to offer a steam key(and shouldn't have removed the option to buy this one) or it's the same game and they should've just continued updating it here.
I mean, I kinda feel like that proves my point about how they should've just done it from the beginning. I get that they didn't know at first they were allowed to upload it on both but they should've actually read the steam guidelines or looked it up? Also, this kinda proves that their "it's a different game I don't owe the people who bought the first one anything" statement was definitely just them trying to make excuses XD.