That's pretty much what it is. I had one of those DVD DVRs back in the day, and they never finalized discs by default since the idea was that you'd mainly use them with the DVR itself, and the device can't know when the right time to finalize is.

The 'correct' way to fix it would be to find the model of DVR used to record them and finalize the discs, but that's pretty unlikely unless the seller also had the DVR on hand. I don't know if there's any regular burning software that would know how to do this.