Is he even that hot of a celebrity any more? I was not in that demographic at all but he seems to have fallen into the weird void of the 'formerly super famous' to me.

Either way it's an old strategy for businesses, pay or convince a celebrity to use your platform to bring their fans along. You want a wider span of 'influencers' though for sure because you don't want Mastodon to become the place with all the Beiber fans.