> It's against the grain to think this way in dynamic languages because variables don't have types, only individual values do.
So? "The only valid type for a conditional is boolean" still works if types only apply to values. Under that principle, anything but True or False value encountered in evaluating the condition of an if statement ought to throw a TypeError, not be evaluated for truthiness. If you are expecting something else, call an explicit, use-case-appropriate function to get the right in-context truth value.
(Note, I'm not saying Python should do this, I'm explaining how the logic applies to Python without any contradiction to the "variables don't have types, values do" principle.)
> IMO every value except boolean false (and, if you insist, nil/null) should be truthy in a dynamic language.
I think that's better than what Python does (Ruby does that by default, with nil included as false, though it is IIRC possible-but-extremely-strongly-discouraged to override the default truthiness of objects so you could have classes with falsey values), and I lean toward preferring that approach, but the idea that a dynamic language would do well to just allow True and False as the only valid (non-error-producing) values for an "if" statement is not, IMO, without some merit.