Yes it is, but the achievement is coded to trigger after the end of the final mission of the playlist. When I played it through with various partners only the one who completed the final mission with me got it.
I read the achievement description and it says, beat halo 2 laso campaign playlist. So......
So I've been running halo 2 SLASO for the achievement, I left it on gravemind and then the big update came, I downloaded the update and...IIRC the achievement is for beating the final level of the playlist (otherwise all the people giving the checkpoint away wouldn't have worked). If the post update is letting you play on I would run with it.
The Playlist mission counter is at 0 meaning that it got reseted, BUT when I press play, it takes me directly to the last checkpoint I had on Gravemind.
Does someone knows if I continue playing will it count for the achievement or do I have to do everything from the very beginning of the playlist?
Plus plenty of people on True Achievements had the final checkpoint saved and could give it to people so they got the achievement without doing the work
The same thing happened to me when I was doing Halo CE laso. After the update, I went to playlist->Resume, and it sent me to the beginning of a completely different playlist. I freaked out since I had gotten to TB, but I manually went to the laso playlist, hit resume and it sent me to my last checkpoint. I still got the achievement when I finished the remaining levels, so you should be fine.
Also I'm pretty sure the checkpoint cheat was patched a year ago.