I wouldn't think that a tensioner is very likely to wear or rust the way that the recommend, but then I haven't exactly had the amount of experience that they have.
My idlers and tensioner felt fine when I replaced my timing belt, but (now that I think about it) I haven't looked to see if they were replaced with the first timing belt change under the previous owner of the car...
If you trust your shop, then just keep trusting them. Look at it this way: you can't do the car by wrong replacing these parts.
