Another line that only hit me on the reread.
As Adam and Blue have this intimate, post-dream-tree moment, Ronan is "staring at them, raw". Ronan explains later, in the Raven King, that the visions in the tree are:
"Nightmares", "Worst-case scenarios. Whatever mindfuckery it thought would be most likely to mess you up the next day."
So maybe that highlighted line isn't only a reference to Ronan understanding the tree because he dreamt it. Maybe his understanding, his rawness, is because he's actually staring at a real world version of his worst-case-scenario-nightmare-mindfuckery: Adam with someone else, Adam with a female someone else, Adam being 100% unavailable to him.
Maybe he sees the intimacy of that moment between Blue and Adam and realises it’s an intimacy he wants for himself but thinks he can never have.
Maybe it dredges up his feelings of unloveability and abandonment.
Maybe, without even stepping into the tree, he's having to face the truth of his sexuality and the impossibility of ever sharing it with someone he has feelings for, no matter how many dollies they drag each other around on.
Maybe he doesn't quite realise all this right now, but maybe it's why he's so raw.