Well, they are not entirely wrong technically, however, the whole reverse lookup process would be incredibly awful and convoluted to do to the point of not only being impractical, but also borderline impossible.
To find alts of someone else all you have to do is blacklist someone, and then constantly look around the world for characters that are not the blacklisted behaving like the blacklisted one, taking notes. However, given how much effort one singular actor would have to do that, and the fact that you would have to be incredibly lucky to stumble upon someone elses alt (considering the amount of servers / maps said character could be), I'd say this case is negligible at best. Especially considering we are currently dealing with a handful of automatic scrapers being able to catalogize every player account + their retainers.
ETA: no, I am not advocating to do these things. going lengths like that is actually creepy and anyone who considers that should reevaluate what they are doing there.