Obviously it’s just Rei lashing out. She’s feeling weird shit and having Jadeite around is making her sixth sense go all wonky. Usagi is undeniably the most powerful thing there and Rei can already tell and she reacts instinctively. She doesn’t think about it for a single second, because she’s Rei so of course she doesn’t.
But I love this subtextually Usagi being here is the beginning of the end for Rei. From this moment on, she’ll never be the girl she was. It’s not the physical danger. That — as we can see — she’s more than ready for. But personally, emotionally? That angry, bitter, isolated and protected person is about to die a slow death. And it’s terrifying, more than any monster or mini-boss could hope to be.
We laugh because Rei’s mistaken Usagi for the most dangerous thing around. But in reality, for the person she is right this second, it’s actually the truth.