Steam version said something about the Publisher tricked the developer in giving them over everything because the dev is bad at English and now the dev doesn't get anything from this game. By buying this are we supporting theft?

That is a rumor that was never confirmed, but Pixel (The creator) has stated several times on twitter he gets revenue from cave story. I can't confirm if it's from all versions or if it's only from the ones he worked directly on (AFAIK, he didn't work on the switch port, for example).
The thing is that the story about Nicalis stealing the rights are unconfirmed rumors people believe in firmly due to the other pelemics that surrounded nicalis and its director a few years ago (they think if they did some questionable things, suddenly all crimes they are accused of are true). However, no one has enough info about any of this, just a lot of speculation with theories about why Pixel and Edmund Mcmillen (Binding of isaac creator) started self publishing or working with a different publisher, but no info due to silence relating to contracts and the like.
I'm not defending them, because the things they quere accused of and proved were nasty (cases of racism and ghosting of several indie creators, enter the gungeon creators among them), and never liked them due to their full drm stance (which they seem to be changing their mind right now), but the fact is we all lack information about the rights theft, and Pixel seems to be in good terms with them, promoting their cave story merch and the recent christmas short game.
So buying the game or not is just up to you