He is 100% incorrect and misunderstands both squatting and castle doctrine.

When there is an issue with squatters, it is because their presence has become a tenancy. It is not much different from overstaying a lease or a friend who crashes on the couch for too long. They have a legal right to be there until formally evicted.

Furthermore, this convo revolved around legal hassle with getting them our, right? Because you can't just throw them out?

Even without knowledge of law, it makes no logical sense that you are not allowed to force them out and lock or barricade the door, but you are allowed to shoot them. Castle doctrine does not protect the deadly force of a firearm more than lesser force. If you cannot put hands on somebody, you cannot shoot them.