Here's the way the specifics break down, and it has nothing to do with 'never' and 'non'
Go read Unyielding Anvil again - it doesn't make the unit a scoring unit 'period', it allows it to claim objectives 'as if they were Troops' [later FAQed to be 'as if they were scoring units']
You know what else are Troops/scoring units? Grey Knight Terminator squads are Troops.
But they can't score for us, because they're Desperate Allies.
So if you make other GK units to claim objectives as if they were scoring units, they work just like any other GK Troop option - unable to claim an objective because they are made non-scoring and non-denial because they're Desperate Allies.
Also, Codex doesn't trump main rule book.
The More specific rule trumps the less specific rule (usually this *does* mean Codex trumps rulebook, but it's not always the case, especially with edition changes) If codex always trumped rulebook than Grey Knights could never take Allies due to the rules on page 81 of their Codex describing their Force Org.
So, the order is, the Grand Master makes some of his units scoring units that might not otherwise already be scoring units.
Desperate Allies makes all of his scoring units non-scoring units.
So, whether or not they have the ability to score like Troops (either by being Troops, or the Anvil effect) they remain unable to score because they're Desperate Allies.