If you mean, "why can't we say that it's a complete waste of time to suppose that" humans are "thinking or intelligent," then yes, I think it is a complete waste of time!

If there's no testable definition, there's no way to say the statement is true or false, nevermind what the implications may be.

It is the same as saying we're all goblethorpy.

It is an absurd question even in the abstract: "prove that you're thinking" ... yea we all have an idea about what that means but it is untestable and it is why this kind of philosophical assertion gets endlessly debated with no real progress.