Unless your mechanic spent quite a bit of time removing the fan shroud, fan assembly, timing belt covers and various other stuff he didn't see the timing belt, he looked at the serpentine fan belt and didn't know the difference. If that is truly the case find a different mechanic that has experience working on diesel rigs, VM Motori engines in particular or they will probably mess up the engine.
I have performed this same belt change on the 2006 2.8 VM diesel sold in the Jeep Liberty for the US market several times. The 2.8 in your JK is a different design from the Liberty so the process is similar, not exactly the same. Heed the advise of "get the kit", get detailed instructions or make sure your mechanic has them or better yet find some one who has done this, don't let them learn how on yours. Not sure about the 2.8 on a JK but on an 06KJ it is an interference engine, meaning if you loose that belt the valves hit the pistons, bend or break the rockers and trash the engine instantly, major cost to repair. Belt change interval is not an option and for goodness sake buy a quality timing belt from a reputable manufacturer.
Don't cheap out and just replace the belt. All moving parts that touch the belt should be changed while you are in there. Every idler pulley and ifits similar to a Liberty, the water pump also. A new belt that runs on an old bearing is a bad idea, if the bearing fails and the belt jumps time it is just as bad a breaking.
Here is a video of the JK change process from youtube, not my video, but watch it and you will see that nobody can do a "quick" check on a JK diesel timing belt.
VM makes very good engines, and has for years. My daughter traded her 05 2.8 KJ off at 260,000 miles still going strong but the interior was falling apart. I have 126k on my limited model and it will probably outlast me.
An average mechanic with instructions and the special timing tools can do this job but without experience or instructions the odds of success really fall.
