They are getting it for free though? They're just having to wait an extra 2 weeks before recieving the update on itch.io.
I could understand your comment if Mike was deciding not to update the Itch version and he's claiming that the steam version has more, so that's why its a different price and you're having to pay more. However that isn't the case. He's willing to give people the update worth $3. Yet he's not willing to simply just instead give over a steam key. Which in turn just seems like he's doing it to get people to buy the product twice. In turn essentially punishing early supporters.
The reason it's leaving a bad taste in peoples mouths is because it shows that being an early supporter and supporting the product by buying it early on Itch.io. Means you get nothing in return except a 2 week delayed update and people like you telling them their opinion is unjust despite it being a solid reason to be upset.
It's extremely common for developers to release products early/early access/early alpha for lower prices. To then give the full release for free to people who supported you early on. It's strange why that very normal practice isn't being done here. Maybe there is a solid reason but he could have stated a good reason in his post.