It's a diss because damn near ten years later, they're still talking about it. It's over, and they still feel the need to mention Pac in a questionable context (and just the fact that it's questionable is already crossing a line). The whole reason the beef started was over a questionable line on "Survival of the Fittest". This isn't any different, and it's not like Mobb Deep was saying anything else when Pac was alive, not on a freestyle, not on a mixtape, nothing. Not until AFTER Pac died. "Drop a Gem on Em" was after Pac died, but as a hip-hop fan, I'd be somewhat inclined to understand that one diss IMMEDIATELY afterward since it's their reply to Pac's numerous disses, just to get their response out there to stand up for themselves. But once is enough, and motherfuckers don't need to keep standing up against a dead man nine years later.
Take the Death Row-Ruthless beef, for example. Both camps dissed each other on wax, but as soon as Eazy-E died, the Death Row artists didn't really bring his name up... not Dre, Snoop, Daz, or Kurupt, the main guys dissing him. And if they do mention him, it's in a positive way, like how Dre talked about Eazy on the 2001 album. But Suge, on the other hand, throws out little disses to Eazy (like when he appeared on Jimmy Kimmel Live).