This is not unique to millennials. Go back to every retail-participated bubble and you will find this condescension (and deservedly so imo). The stories of strippers in Vegas buying multiple McMansions in 2006 come to mind.
You points about relative return measures (ROI) having large base effect issues miss the point of what's going on: it's not the magnitude of the swings which are notable, but rather the circumstances around which they're occurring.
Enron was a massively complex business, and the true value of its assets was thus a massively complex question. HTZ does not enjoy this same conundrum. Other financially impaired but high name recognition names have had similar behavior (AAL, DAL, to name a few). Robinhood have lowered the bar to trading, that children are now "playing" the markets when they get bored of Minecraft and Fortnite (this is not hyperbole). And then you have one strata above who are likely the real problem and have been active in every bubble in recent memory. Financially unsophisticated adults have indoctrinated with "buy-and-hold" from the likes of Warren Buffett without understanding the edge cases likes bankruptcies (incidentally, Buffett sold all his airlines holdings which are now a retail favorite).
Simply put: people don't understand what they're doing and that's what makes this so amazing. They think "people aren't going to stop renting cars" or "people aren't going to stop flying" and so they buy the stock, cocksure that over the long-run they will be rewarded. The future outcomes of HTZ common shares is pretty certain, and not at all consistent with current retail behaviors.