This is something I did think of too. On one hand, having to buy it again is usually a terrible thing to have to do. Usually triple AAA compnay kind of business practice but that overshadows the actual reality here:
Itch.io + Steam, at full price, would be $5.98 total. The literal worst case scenario here. There are many worse games that charge exceptionally more even when on discount, never mind at full price from both indie and triple AAA. And this is from one dev and this dev isn't failing from lack of trying. Steam - limits keys, Itch.io - refunds must be made within 90 days of purchase (if you wanted to repurchase it on steam). He tried but got blocked on both ends so I can't blame him.
The only thing I'm not too happy about is the game now requiring steam to run, as opposed to running independently from any launcher.
A small nitpick that can be worked around but it was a feature of the original, intended or not <:D