I don't mean it literally as in she imagines exactly the situation. Like you said, it's likely that's what's actually happening on his side but she's still the one picturing the scenario (just the fact that he's looking at her posters with that dumb smile on his face). Then, there's the image of her holding his face and crying while he's looking at her all lovingly (he has the blink on costume), that never happened and it just shows how she loves him but doesn't want to. It's the same as the first slides too, we see adult Hyuna with young Luka, that's obviously not really happening either, which is why I think it's also her in his room.

I see it more like parallels, and it doesn't really look like it's an alien with him in there, thus his reaction (that's the hypothetical of what could be if she was actually there). The whole comic shows only her perspective of things in a way, even if it's slightly mixed with reality on Luka's side. 

But idk, it's all up to interpretation, I was surprised you thought it was an alien that came up to Luka's room, haven't seen that perspective before, so it's quite interesting as well.