I don’t know where you got that number from, but at least these statistics say Mastodon got 9 million users (but not all are active off course, same should be true for BlueSky): mastodon-analytics.com And this account claims 15 million: mastodon.social/ and a Wikipedia article says “On 19 March 2023, Mastodon passed the ten million mark for registered user accounts”: en.wikipedia.org/wiki/Mastodon_(social_network)#2…
Now, I do not claim these numbers to be correct. But compared to your estimation its vastly different.
Edit: Just for context of my reply, as you edited yours. You said you don’t believe Mastodon would even have 100 thousand users.