metaclasses are a red herring, enums are just effectively final, so this is safe to do

Are you sure it's not about the order of operations? I.e. first __bool__ gets checked, then __len__ and then the same thing happens for the metaclass.

But here __bool__ gets checked and then immediately it goes to the metaclass.

I think falling back to the metaclass is correct, it just happens too early.

Alright I had a read of the blog post, maybe metaclasses aren't involved in truthiness after all and it just so happens that the default is to return True if you have neither a __bool__ or __len__, although I'm a little confused about the purpose of __bool__ on EnumMeta, when that is the default truthiness anyways.

I double checked, looks like the metaclass does matter, but only for the truthiness of type[Enum], not the instances.

>>> from enum import EnumMeta
>>> EnumMeta.__bool__ = lambda self: False
>>> from enum import Enum                                                                   
>>> bool(Enum)
False
>>> class Foo(Enum):
...     a = "a"
...     
>>> bool(Foo.a)                                                                             
True
>>> 

That's right. It would also affect bool(Foo)

... continuing from your monkey patch ...
>>> bool(Foo)
False
>>> type(Foo.a).__mro__
(<enum 'Foo'>, <enum 'Enum'>, <class 'object'>)
>>> type(Foo).__mro__
(<class 'enum.EnumType'>, <class 'type'>, <class 'object'>)