Alternately make non-troll leftists join a non-troll instance if they want to just go out into the wider Fediverse without the stigma. Once they federate they won’t have to be on the CTH-successor instance, they’re choosing to be there. Like I said in my other reply to you, I don’t find the lemmygrad users nearly as trollish as CTH was. It’s not the ideology that’s the problem, it’s the trolling, and they’re pre-announcing they won’t do anything to curb trolling by their users.
CTH wasn’t just a sub that happened to have some users who also trolled, they frequently organized and bragged about trolling and brigading there. And while mods need to be ready to ban individual trolls, it’s also a volunteer activity and not dealing with a rotating cast of trolls or massive brigades could be worth shutting out some posters who wouldn’t be a problem themselves. I don’t think it’s an unreasonable stance to say “wait and see if it’s a problem”, but I also don’t think it’s an unreasonable stance to say “we know what’s likely going to happen, we’re not robots who have to pretend we were born yesterday”.