I suspect (and this is just a guess) that TPE said they would carry Northern passengers, but not Virgin passengers. Staff at Man Picc incorrectly advised everyone they could use the TPE service, when guard realised he announced the correct procedure, but avoiding conflict, didn't check any tickets, but relied on everyone to correct themselves. Making the best of a bad job. But staff are only human, and do get things wrong.
I was once on the 09:35 XC ex Penzance for Man Picc, but the following 10:00 GWR to Paddington was cancelled, so XC were taking passengers to Plymouth (as long as they were at the station 25 mins early!) However, the Train Manager came on around Cambourne, saying the GWR service would now start at Par, and all GWR passengers must change there, even if only going to stations before Plymouth (again, not any permitted fares or routes, although he didn't make this totally clear. I guess, as his four coach voyager was heaving, he wanted as many off his train as possible!) He did make very clear anyone on after Par would be required to buy a new full fare ticket if only valid on GWR. And he came down checking. He was also insistent that XC passengers had their seat reservations, and that GWR passengers would have to stand.
It does show though, that advice can change too. What is valid when you board can alter during the journey.