wangus,
For the test, the examiner wanted you to use the "official" primary nav-aid, which is LZD and the DME from ILDY as published on the chart.
For GPS approaches, the "official" primary nav aid for the approach is the GPS, so it is ok for the approach.
Next time, do what you did (display both the GPS and classic navaids, good airmanship), but reverse what you call "primary" and "secondary" (say "I am on QDR 353 and 4.4 DME, which is ROMTI, confirmed by the GPS").
As far as the "why can I not substitute GPS for NDB" is concerned, there is no polite answer. In the US you would have been able to use the GPS overlay to the NDB procedure including the hold, even without an ADF on board, provided that
1) the Approach is not labeled "GPS Overlay not authorized"
2) you have loaded the approach data from the database
3) you have done the necessary RAIM checks, as for any GPS approach
(PS - you DID check RAIM for the GPS approach, didn't you?)