Emotional dysregulation is an acknowledged trait of ADHD, "rejection sensitivity dysphoria" appears in some discussions but it is not officially recognised anywhere as a symptom or disorder or anything, but it can be seen as an instance of emotional dysregulation anyway.
Description has to come first, then we can form hypotheses.
I can't respond to your newest comment but essentially 'rejection sensitivity dysphoria' is not in the DSM-V, so to acknowledge it one needs to have both a definition and some reasonable experience with the diagnostic entity (i.e., it's not enough to read about a disease to be able to diagnose it).
That doesn't invalidate the parent's point. We can still question if the details and way it is presented is accurate.
I should've been clearer, I was replying to the "so then how can we make definitive comparisons" point.