I think you don't understand one fundamental trait of "OOTS leveling": NPCs don't need to actually level up. They can be paradropped in front of the PCs any time and when that happens, they'll magically be at the "right" level, appropriate for the story and challenge.
So, regardless of what we do or do not know about Tarquin's background, he's clearly a fair bit higher level than the Order. I had called that before we knew Durkon's Holy Word hadn't affected him -- that piece of confirming evidence wasn't even needed to know it, because it's so obvious that it (should normally) go without saying: when the PCs meet a new big bad villain, the said big bad villain is more powerful than them, no matter how many years of adventuring he's got under his belt.