Check this out yo:

In [1]: from typing import Sequence
In [2]: class DumbSeq(Sequence): pass
In [3]: issubclass(tuple, DumbSeq)
Out[3]: True

This has to do with the implementation of GenericMeta, where our __subclasscheck__ ends with this line:

        return issubclass(cls, self.__extra__)

And for the container classes in typing, they're defined to have the collections.abc classes as their __extra__.

Pondering a fix to suggest.