1. The OG Merlin and Heinrich Kemmler for two, there is a huge summoning circle of Demonreach, so with time to research and prepare you can bring the Big Bad to the island that way. Harry had neither for Ethnui. Baelor, Ethnui’s dad for one.
2. The earlier possessions by Nemesis we have seen were of already supernaturally strong individuals. This is the first vanilla mortal.
3. Justine was infected when Maeve died in Cold days, the Mantle went to Molly, Nemesis went to Justine. Molly had been trained in mental badassery and had even beaten Corpsetaker, Justine had a fragile psyche. Previous hosts were either willing like Maeve, or invaded by a prepared vector (like Lea), the invasion of Justine was neither prepared nor willing.
4. It was a clue that Marcone was also a Knight. The winter Knight Mantle was patterned after something, most likely the knights of the Blackened Denarius. I be the Kights of the Sword could also manifest a banner in the right circumstances, both Marcone and Harry considered Chicago ‘their’ City to defend, Michael could probably have manifested a Banner during Battle Field as well, it is his city as well.
Butters a servant of the City (and Ill served by it no, he probably could in San Diego during Comic-Con though), Sanya is a citizen of the world, no local connection to the people in it, no banner.)
5. Marcone has one of the most formidable wills known in a vanilla human, he has even turned down Lara’s come hither. That will, that focus with even the most marginal of talents would be enough.
The Knights were there not to stop Ethnui, but to stop Harry from becoming a monster in his own eyes, his own greatest fear, which had previously led him to try to kill himself. That was the role of Murphy’s death a goad to his humanity unlike any other, the sheer outrage and the instrument behind it being so loathsome. My view Lucifer was behind Rudy’s lax trigger finger to make him a Starborn more in his image. It’s why the Knights were there to intervene.