http://www.autozone.com/R,APP1111329/vehicleId,2386801/initialAction,partProductDetail/store,1140/partType,01155/shopping/partProductDetail.htm This is the Web link to Auto Zone. It shows 3 timing belt guide rollers and the tensioner roller.
The timing belt tensioner is a spring loaded piston which has engine oil pressure acting on it when the engine is running. The tensioner pushes against the roller which pushes against the timing belt. The Chilton’ s Repair Manual does instruct to remove this tensioner and test it under varying loads. The repair instructions the shop used (or, not) should have included similar instructions. They may be using any of a number of available manuals: alldata.com; mitchelonline.com; the official Subaru Online, or DVD based manuals; etc. You need to find out which, so that, when you cite a recognized authority, you’ll have your gun loaded.
I think that the bolt through the center of the tensioner roller wasn’t tightened correctly. When the engine ran, the roller tensioner couldnt tension the belt, and the timing belt jumped some teeth, and the pistons impacted the (now) out of time valves.