You need to explicitly inherit from an ABC to be considered an instance of it. With Protocols you just need to match the interface, duck-typing style.

(You can also do this with ABCs by writing custom __subclasscheck__ and __instancecheck__ functions, but those can't be statically checked, they only work at runtime)

AFAIK protocols actually use ABCs internally(?) Just in a more constrained form that static type checkers can understand.