The issue wasn't whether the serve was returnable or not. The issue was that Lahyani ruled that the call came after Tsonga hit it. Tsonga said it came at exactly the same time. It's a tough call, but when it's that close, I think the benefit of the doubt should go to the receiver (Tsonga). But, the sound is slightly off of the video, so it's really hard to tell, and if the umpire feels that the call came after the hit, then it's not a "replay the point" situation.
I don't think Djokovic would have argued the call if he called a let.