Tensioner?
Should have been replaced anyways as part of the cost - so this is a "cleaning" I too - don't appreciate.
As a delivery driver, I've seen many "kits" - these tensioners come standard in them so I don't know why that they needed a part separately - the parts required, usually come as a required condition of the repair - not optional. (They do have bearings in them and so that makes for a limited lifetime - so it's required.)
It's also why I like forums - for the participants leave behind their experiences like footprints to follow a trail to get to the destination - only the journey is a small book they leave behind for others to read - we all get a little smarter about it because of that effort.
It's getting to a point that Warranty is not a thing anymore to be used to enforce the shop to stand by their work, it seems more like a passing notice that you should do this, but it doesn't always mean it's required. That is the part I don't get, definitely fishy.
And also good to know that many a shop - to save time and turnaround costs - will drop specific items from the list as long as the main repair is completed - only that small detail can put you back into that shop to have them fix all their work plus repair the part that caused the fail due to it not being repaired/replaced at that time because of lack of time to wait for it to arrive.
It's like a time bomb they put in your engine - they have it set to go off, only you're not sure exactly when it will happen, only that it will and do you want to be there when it does?