Uhmm I don't think it's done with my old bf2fp tool, it's highly probable that it's a private solution like hooking the game information port or the outgoing heartbeats and so on.
Anyway it's stuff on which I worked 10 years ago so I only wanted to give you a reply just in case nobody else will do it.
If you want to go back in the days:
http://old.zenhax.com/bf1942-bf2-bf2142-t44.htmlThere are also other topics in the same Fake Players Bug section of the forum (it's an html-only archive, not a real forum eh)