Here are some contenders for said tweet:
"Are CVs of failures still a thing? Because I just got enough material to start mine 😎 #POPL2021"
"brb changing my spooky display name from "{-# OPTIONS --type-in-type #-}" to "[POPL ‘21] Rejected submission""
"Happy Mid-Autumn festival, first day of Halloween, and POPL rejection day!"
@ionathanch That’s how you shitpost about rejection. =)