I don’t know, the line “at this rate, the only original part of this thesis is your name” implies the issue is not just failing to cite sources, but having no original thoughts or contributions - maybe it’s exaggeration for humor’s sake, but I definitely did not read this as simply forgetting to cite sources. Regardless, it’s so flippant and playful it feels like it was written as a joke, maybe even written to make a meme, hence it doesn’t feel like a real email.