> the notion of commutativity here is very weak and counterintuitive; it only seems to cover the applicability of an auto-merge heuristic

This is completely false: in Pijul, any patches that could have been produced independently can be applied in any order without changing the result. There are 0 heuristics in Pijul, unlike in Git where even random line reshuffling can happen (there are examples in the "Why Pijul" section of the Pijul manual).

Obviously, deciding whether a merge has the correct semantic is Turing-complete, and Pijul doesn't try to do any of that.