Hiding under another name is allowed. Impersonation doesn’t simply mean to use another’s name, it means to use another’s name with intent to deceive.
Much like IA, intent and context are significant.
That’s the difference though, the players in this theme are not saying “maybe we’re actually the mods and maybe they’re not”. They are saying they are not the mods and that it is a theme.
They are not “making all think they are another player”. If they were do that, it would be blockable, but they are not doing that.