GP is not talking solely about ADA though, they are asserting that there is a fundamental difference which is embodied solely in the presence or absence of a return value which is missing from e.g. "everything is a method"

Differentiating between pure and impure functions might be useful[0], but while not strictly orthogonal the presence of a return value doesn't tell you anything about that. Even ignoring the error signalling, read(2) has a return value (the data being read) and also has side-effects.

[0] though it's debatable that this distinction is really useful in and of itself