hm that is odd because the devs themselves stated that you would have to buy it both separately
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What i said was that there is two types you can buy on this site, and one of them gives you only a Steam Key, while the other the game for Itch only.
There is no way to buy the game for Itch and get a Steam key included with the purchase.
Ofc right now they not available for purchase because of issues with their itch store, so its tempoary down.
So if they want to buy it for Steam, they have to do it directly at Steam store.