The TSR thread is bizarre. It would be grave for Pearson to misrepresent the data breach; what they have described should not affect scores.
Nothing is impossible but in the first instance I lean towards the majority view expressed above.
Two things immediately come to mind. First, I wonder whether everyone is aware that TMUA marks were rescaled in 2024. Old exams being taken as mocks would have been marked with a higher score than they would earn under the new system
Also - and we see this at university all the time - when students mark their own mocks they can be very casual about it. If they are sure they have the correct multiple choice answer, they may not even be checking it properly, just totting up the marks. Some of the mock scores may be inaccurate , because TMUA multiple choice questions have wickedly deceptive answer options.
A bit of explanation might be a good idea but the cynicism because these pupils are predicted three or four A stars is really inappropriate. The point of TMUA and similar exams is to distinguish within this group.