Well, here you presume person A had to wait for person B. The advantage with mod development is that you can immediatly use the main game content and start modding. That's how I imagine the Level Designer would operate. But now imagine if a Developer was working on a map and then suddenly vanishes? Another team member would need to pick it up and learn what the other guy was doing. This was rare but there was one instance where a Dev left and the Chapter he was working on had to be scrapped and redone by the other team mates.
Wow, and I thought that would be obvious. Of course they're taking the original level as reference to stay true to the game, but they're actually reimagining the game world and if needed extend the levels or even cut them shorter!
I've been following the mod almost since the beginning, I knew just how massive this undertaking would be and how much work was lying before them.
It's not already handled for them. They remade it on a different engine alltogether, a more complex one and a more powerful one in the way it gives you more possibilities. New code, new content. The very goal of the mod was always to use the full capabilities of the new engine and they even achieved new technical milestones. One of which is the Facemaker or the very detailed skybox you can admire at the end of the Official Trailer.
Also a note: The project leader stated the project as you know it now has been in production for just 3 and 1/2 years, that's when he took over and got the whole thing back on track. Sections had to be redone, several developers left.
Also the community is contributing to a Wiki, if you're iterested you can read on: http://wiki.blackmesasource.com/
EDIT:Ah there's also a comparison shot I particularely like:
Back when the project started, it was still using the stock HL2 content:
OLD Office Complex Shot
The current still WIP shot of the same place:
NEW Office Complex Shot
To be honest, after seeing this I don't mind waiting some more. You know it's the state this thing comes out that will be remembered. If it feels rushed, unfinished everyone will remember that.

Starkrun here on public Computer:
Well the mod IS done, its just getting some finishing touches, script smoothing and a few here and theres. It could easily come out and they patched after the new year but they want a solid mod out fo this. Once released the bugs will be found with the THOUSANDS that wiill be playing it.
I fear though that Nico might have won his GOG on this... Oh well, it was a god bet and ya know what... i till feel good, and I'm Still beating on DUKE so BOOYAH!

Please tell me where you'd read this was done. I'm curious.