> That was true in 2011 and it is still true today

Python's methodology greatly shifted since 2011. For the better or worse, Python lost some of its duck typing. Nowadays, most people use linters. Wherever you'd try to pass sre_parse.SubPattern, the linter will throw an error saying it's not a Sequence even if it fully behaves like one. You can silence that error but you'll continue receiving such warnings all over the code, whether in stdlib or out in the wild. The meaning of Sequence now shifted to "inherits from abc.Sequence". The only thing wrong with PySequence_Check is that the ecosystem shifted, but its view of a Sequence remained the same.