It's not the worst idea. Tons of software goes through "closed betas," or "canary builds," or invite-only phases like this to test at scale, in a production-like scenario, but without opening the floodgates and getting inundated with issues reported by regular people. Heck even Fediverse websites sometimes close their registration pages if they're getting more signups than the admins/mods can handle. And that makes perfect sense, the last thing you want is to suddenly become unsustainably huge. It leads to a kind of social rot: trolls run free without enough mods to stop them, spammers run rampant, etc.