Yeah, it's simply not true and beyond their power. You bought the box as part of your original contract. It is yours. They can't suddenly say "oh, this box that you own is being loaned to us". That applies only to new equipment that they loan, as part of a contract after that date - for example, for a TV Box Pro required to access a 4K subscription.
To put this into context: BT has recycling goals, but they also realized that for a lot of people, having an existing box that can record Freeview means most don't actually need the TV service. So they started loaning them instead. This probably also means they can capitalize and depreciate the boxes which is better for them than a one-time expense.
Example: We have a BT TV Humax DTR-T2100 box and we were asked to return it at the end of a contract, but we said "no, actually we own this box, it was sold to us outright at the time", and that was accepted. We left BT, at which point after resetting the old BT TV box it worked as an unbranded YouView box. We rejoined BT TV a few years later, and now have one BT TV Box Pro, which is on loan. Both of them now work as BT TV boxes and can record shows, but only one of them can access subscription channels at a time, as that is what our NOW/Netflix plan allows. If we were to leave BT TV, we would need to return the Pro box, but the other one is ours until it breaks or we sell it.
