Thanks, HN39, I'm suitably "astonished"!
Will assume the BEA is right on that point, at least. So the horizontal data (TRK & GS) come from the IRU part of the ADIRU, and the vertical speed is simply the VS supplied by the AD system part. I had overlooked the fact that pressure-derived VS indications moved on a bit during my career, in terms of lag (lack of), as well as accuracy and reliability.
I should not have inferred that you were suggesting the horizontal data was also from the air-data (AD) system – my apologies.
But I agree with you that it seems odd that the displayed FPA/FPV ("bird") does not, apparently, derive its vertical datum inertially, from the IRU.
The rest of my comments still stand to be corrected...
Chris