Your argument is that they were sorcerers and then became wizards?
The way priests are moulded into the image of their God makes that very unlikely indeed, to the degree I didn't even consider the possibility. They'd also need to relearn everything they knew from scratch, create massive amounts of magical infrastructure while having lost their own magic, and develop a magical language that suited their mentality from the Old Ones' and design spells based around intellectual control and understanding of the Winds rather than on Faith. Even if they had theoretical knowledge for some reason, they'd still have to learn how to apply it, which would be very hard.
There is very little overlap between the skills and talents required to be a sorcerer and a wizard (it's much, much 'easier' to cast spells as a sorcerer, most of what you absolutely need is faith and inherent talent).
The experienced ones would also only want to do this in the first place if it was consistent with their goddess to want to do so.
Note how I began by saying that the 'false' Truthsayers weren't necessarily corrupted, not that they certainly weren't. When I mentioned the 'fact' of Danu answering their prayers, I was speaking from the assumption that they were actually Truthspeakers with a Truthspeaker's powers, rather than sorcerers of another god or wizards, as they'd have different powers. It's possible, I suppose that what you call Comprimised Truthspeakers are actually not Truthspeakers at all, but instead something completely different, but the difference should be very obvious to the Sidhe, as Ran could tell when Sorcha was drawing on the warp, so the Sidhe would be able to tell that the 'false' Truthsayers were not, and were instead using a different Lore, as that's something that's immediately visible to someone with Witchsight. They'd also then know what a wizard was, and how they differed from sorcerers, as they can directly see the difference when the spells are cast.
Now, of course, this might not be canon Warhammer magic, but I'm not sure that we've seen any reason to think that so far.