Because I was born on the internet and raised. Well, I know how people are so he is either on XIV or a private sever. Hes exactly the type of person to say something like "omg I hate that SE did x, im quitting XI and will be on XIV, they wont get my money!"
I mean while that's entirely possible, it's just an assumption. You basically made an assertion of fact which implies you know they are on a private server, either because they told you or because you've seen them on one and can confirm it's the same person, or saw them live streaming, etc.
You can say "they're probably on a private server" without knowing this, but you can't say with certainty that they are.