Everything in the universe is either a potato or not a potato.

Is this a negation, a true statement, or valid statement?

(I was sort of joking when I began this comment, but now I'm genuinely wondering.)

This one threw me off too. But then I remembered from an earlier video that you can only use the parameters from the question. The question says that all turtles are ninjas. In the question's universe, all turtles are ninjas (because turtle is the subset of ninjas' superset). There is no way to be a turtle without also being a ninja in this scenario.

It is possible to be a non-turtle ninja. You're right about that. Master Splinter would thus be in the superset group of ninjas, but not in the subset group of turtles. Basically, being ninja is necessary to being a turtle (all turtles are ninjas) and being a turtle is sufficient to being a ninja (all turtles are ninjas).