Sitri refuses to accept the reality about the MC's existence. He will only accept his fantasy that Solomon is alive and back in Hell.
Knowing and accepting mean two different things. Maybe he knows that MC and Solomon are two different people. But, accepting involves emotional acknowledgement of something. You have to make peace with it, even if it's painful.
Between fantasy and reality, he can only accept one because they contradict each other. If Solomon is alive, he can't have a reincarnation. If Solomon's reincarnation is here, Solomon can't be alive. The one he chooses to accept is the fantasy.
In other words, Sitri chooses Solomon over the MC. The fantasy of Solomon is more important to him than openly (and CONSISTENTLY) acknowleging the reality of MC's existence. He continues to indulge and protect that fantasy by calling MC Solomon, and getting defensive whenever someone (including MC) tries to tell him to truth.
I mean, I get it. Fantasies are better than reality, but it's so disrespectful to the MC. Especially, to indulge himself in that fantasy while in the H-scene with MC.
