It's so contrived, yeah. "I'll die today kthxbye lol" isn't the same as saying "brb fixing plotholes in another world, and it's all because I sort of froze Yuuko in life gazillion years ago. oh, and I'm dying when I'm done". The guardians are smart, they had the right to know. I mean, why are you even surprised Yue is so pissed at you? (And you know, he still doesn't know.)