Comment by redeyedtreefrog

I think the point in the first section is that the way ABC instancecheck/subclasshook interacts with pattern matching is surprising for anyone not familiar with ABC. It allows you to check for a match with arbitrary functions, beyond simply checking if an object is an instance of a given type. In the final section where he has issues with caching, I presume he hasn't read about instancecheck, your code would fix his issue.