You are almost on to it, the “it” being the flaw in the study.
The study has it backwards, at least for some people: If one drinks heavily after hours, then they are more likely to feel like (insert expletive of your choice) while at work. If one feels bad, they are more likely to have to force that smile, rather than have it come naturally.
I exclude from the above: Natural sourpusses (of which we have quite a number on FR) and those who come to work “loaded”.
“Natural sourpusses (of which we have quite a number on FR)”
tons of tiresome zealots here too: tesla zealets, Q zealots, Apple zealots, conspiracy zealots of various stripes, et. al. ...