Online poker is very much beatable. Poker isn't solved in the same way chess is. It also depends on the site and the rake. Some unregulated sites don't do KYC so collusion is possible.
What I meant by "poker isn't solved the same way chess is" is that if you take a solver and follow all the actions it tells you are "best" you will not make the most money. It isn't like stockfish where by using solver outputs you will automatically make more money than the best pros in the world. 99% of poker is understanding the unique ways your opponents are bad and adjusting your strategy to profit the most from them. Even the best pros in the world still make mistakes.