As you say it was simply a disagreement over business, and that's okay all around until ADWCTA took it to reddit and made it a personal mission to destroy Heartharena ("I hope that streamers, organizations and other expert Arena players alike, including Cloud9, will stand with us on this, and not help the programmer to continue to exploit our work product"). That's unprofessional and simply nasty. He got paid what he was promised, and that should be the end of it.
ADWCTA's post is also filled with many obvious distortions intended to exaggerate his importance and minimize the importance of the owner. Read between the lines, and read the response, and it's clear: the owner created Heartharena long before ADWCTA became involved, does all the programming, all the design, works full-time on the project, and puts his money into all the costs. That's not just "the programmer", that's the everything. Somebody with those skills could easily be making six figures salary if they wanted. Meanwhile, infinite arena players and wannabe Hearthstone pros are a dime a dozen. The fact that ADWCTA even thinks he+merps deserve 30% of Heartharena just shows how out of touch he is with reality.
If anything, giving ADWCTA's bragging that he has a "fair share of contacts from the business side", it sounds more like he's trying to exploit the owner by demanding an excessive share than the other way around. He took it on originally as a fun hobby, and then got greedy when he saw there was actually money to be made.
I'm not saying the owner is 100% in the right, if indeed he never offered even a small amount of equity. I'm not an expert on these things but it seems like he would have been better off trying to retain ADWCTA+merps by offering a small amount of equity rather than risking the consequences of a split. But the bet he's making is that either he can keep the algorithm updated himself or contract another strong arena player who can express intuition using numbers, this time hopefully without an overinflated ego. I wouldn't bet against that.