Do they say why they want you to return the equipment? Maybe what they want is to upgrade it and can't do so until you bring the old stuff in to swap it out.
The guy from Comcast, was this someone you independently called and spoke to or was it the person on the phone representing themselves to be from Comcast?
As with your bank email the other day, verify the information independently by calling a known phone number for Comcast and ask them to explain the request for the equipment.
If you are getting numerous calls about it, it wouldn't surprise me if it was a Comcast competitor (such as Dish Network or Direct TV) trying to get the more vulnerable and naive Comcast customers to turn in their equipment.