The wording is daft.
If they complained about "over-reliance on a single means of navigation" it would make more sense.
Taking any two out of map-crawling, VORs, NDBs, GPS and DR should allow the basic cross-checking you need for safe navigation. I can't understand why they single out GPS. Perhaps pilots over-rely on GPS more than the other things, but the error is to over-rely on any single thing.