I never quite hit GM in any TGM game anyways; as fun as it was, I definitely had trouble keeping a clean stack at high speeds.
Afaik Arika is not allowed to make non-guideline Tetris games, but the fact that they made Tetris 99 for the switch has got some rumors going that they reached some kind of agreement and maybe a tgm4 could actually happen again.
TGM4 is essentially finished and just in hold until Arika figures out how they're going to distribute it. I think you're right though that T99 is kinda Arika testing the waters with a console version and I think they might release it on Switch at least.
I have formally dealt with approximation algorithms before and I skimmed the wikipedia article, yet I did not find anything that contradicts me. Note the in the section on hardness, P and NP (obviously) appear as bounds of solutions. Where is the contradiction?
Further, all this (approximation algos and P/NP algos) are discrete, complete algorithms. ML is a completely different story, as one is never certain whether some optimum is global or local. Hence, ML is a heuristic, while Djikstras shortest path algorithm is a series of steps with a success guarantee (with provable worst runtime behavior).
The paper proves that Tetris is NP hard, but that is only because Tetris is infinitely long game. It's still easy for a computer to play as well as a human and faster