Well, it seems to me that Athiel believes some Juliet™ bullshit, so they think the MC knows everything. There's a reason the characters don't talk directly about what happened to El, or why Athiel keeps storming off when others bring it up — so they don't realize that the MC doesn't know. Plus, that would explain why Athiel doesn't understand the MC's grief, or why they advised the MC to date someone else.
I know, it still doesn't look very realistic, but that would make more sense. Or maybe I'm just trying justify my chosen RO's behaviour.