As to gameplay, I can't really give any personal experiences with CoD, but ultimately in esports, with multi-version titles, the version played by top players is whatever version tournaments use. After the move to CoD4, before CoD:MW came out, the community got pretty organised and together managed to take control of their scene, meaning that they managed to resist moving to newer CoD titles which are far worse for competitive play than CoD4.
Actually, iirc, there was a bit of a fight back when they first started getting organised between the "stick with CoD4 and screw future releases if they suck" and "screw CoD4 too, we should all move back to CoD2 and pursuade the tournament organisers to follow us" groups. CoD4 won, presumably as a decent middle ground between suitibility for esports and not being as out of date as CoD2.
Besides, has the COD engine changed since COD4? I'm not sure it has.
(Tek9 is one of my company's sites, happened to see that while testing some advertising stuff this morning. I would link to another source instead, but by the look of it the map is actually being made by a Tek9 member who only announced it there.)
As to why it's CoD4 (a relatively old game in the CoD franchise), Tek9 is a competitive gaming community, and gamers who play in CoD tournaments chose to stick with CoD4 for the gameplay, rather than always chasing the very latest graphics.