It's such a bizarre perversion (heh) of a genuinely useful piece of advice for people (you're only as young as you feel), meant to suggest that just because you're starting to age doesn't mean you have to give up being active and doing the things you enjoy.
You absolutely
can still be jovial and silly as an adult. Having someone who's usually in a genuinely (not forced or annoying about it) upbeat and jovial mood in the workplace can seriously boost morale. But it has to come in moderation, and never at inappropriate moments. I've got a reputation at the office of being a bit of a goof, always good for a clean (or dirty) joke and exchanging HR-unfriendly barbs with my coworkers. In fact the HR woman has a dirtier mouth than the rest of us (obviously it's not a big company). You can still be a professional and a goofball so long as you temper it with wisdom.
But that shit
never comes out when someone gives bad news (death of a relative, someone gets fired/quits, etc.). And it's never carried to a point that it disrupts productive work either. And people still dress like professionals. Polos and shorts are common, t-shirts are tolerated. Button-ups and slacks for special occasions. And as it turns out, "professionalism" is one way skilled workers tend to endure economic downturns (even in high-turnover industries) better than their t-shirt-wearing colleagues.
You can always spot the junior-level employees with one glance because they're always wearing "clever" or "branded" t-shirts with some dumb slogan or a mass media cultural reference on it. The ones who quickly adapt and ditch the t-shirts end up working out just fine. Those who don't tend not to make it very long -- not because of the attire, but because they tend to suck at their job (tardiness, laziness, incompetence, drama, etc.).
I suppose most of the people who reason like these redditors have never actually held down a job of any significance, so the above won't matter much to them.