(Please correct me if I'm wrong in the case of this game)
Devs and publishers often feel embarrassed to say it, but they would often prefer for people to buy the game on their native platform or on platforms like itch.io, since Steam takes 30% of the revenue and itch.io takes only 10% (by default, but authors can even set it to 0%).
I love Steam's integration with games (workshop, achievements, collectible cards), their platform runs super smooth, and you can have all the games in one place.
If those things are important to you, you should get it on Steam, no shame in that, but if you can give those up to show some extra love for the creator, buying them here is the way to go.
It may not be much extra money for the creator if one person does this, but if all people start doing that, then this may really help them out!
On the other hand of this argument, buying the game on Steam may help the discoverability and trade extra cash for better marketing by Steam recommending this game to more people.