These new people already knew where to get this population information or someone was directing them to Phoenix. ~4000 XIV players didnt just "go try FFXI" and all land on the same server.
I do believe that people trying to get into XI out of curiosity in 2026 are going to look up some third party information on places like here or (more likely) the subreddit or Discords, where the narrative for about the past month or two was indeed "go to Phoenix". I imagine the situation was that Phoenix gained that early lead before any big new player influx, just enough of one to make it the default suggestion for people wanting the next Bahamut/Odin.