This is the crux here, is he referring to the tensioner assembly?
Then it may indicate the WATER PUMP will also have to be replaced - this is where the "Tensioner assembly" goes onto to keep the belt centered - if the cover and the subsequent "yanking" and retightening from the jumping of the belt - that action sets you up for a imminent failure of the main device the keeps your engine coolant moving and the engine itself from overheating.
Or you really have more of a fact finding tour and you will have to look into this further, for to me, the loss of "tracking" usually is from the Tensioner losing it's ability to keep the belt tight against the gears and you're noticing problems with timing and "Check Engine Light" is trying to come on.
There is a specific series of steps that have to be performed so you don't lose the timing aspect - including finding Top Dead Center if needed - and when you have such an issue, you really should place the engine in TDC - with the pin at the Crank Harmonic balancer side - in to "freeze" it at TDC - you also have a tool that verifies and will hold the cams in place as you rethread the belt. (shown above)
Now, if the vehicle or motor was from a prior (used motor replacing blown motor) then a previous user had damage and you're now stuck with motor that has bent parts and wobbles - which is out of plumb and causing the issue requires a further diagnosis including a runout tool to find the cam that has that wobble in it.
For the Tensioner, and the bottom gear from the Crank - has/uses a flange to keep the belt to the crank - but it is up to the tensioner to handle and keep the belt from vibrating excessively - which if what I fear is true, then you have a worn (if not just one, both) cams and even a warped crank gear - and your stuck with a motor that is warped - looks like from the shell casing - got struck and froze the motors cams or crank gear - for a moment as it was under load, causing this wobble from the instantaneous torque impact this strike caused the motor to slow or stop and bend something until the clearance was re-established after the impact rebounded.
Or worse...the Crankshaft Sprocket - being as worn as it is after a lot of miles, has done something like this...
Your part number is BE8Z6306AD - Crankshaft Timing Sprocket
So, review the photos above and if your engines tensioner, water pump and CAM-shaft sprockets are all ok, then review and look to see if the CRANK-shaft sprocket gear is failed - the VID to help explain this, is here...