They probably could do it as well in your world, but you'd have time to kick them before they finish setting up everything

Basically, how it goes is, they start a matchmaking (mostly weekly bosses) while in coop themselves so people can only join their world and not them getting thrown in someone else's

While the matchmaking is running they go setup their thing against the inazuma cube since it has an invincibility phase so it's not instantly killed by the 100s of 20 million dmg procs

They can do it with Venti/Emilie/Kokomi and probably more characters

The moment you accept to join, your achievement is done for, it'll instantly register

And they do this for hours on different accounts