I think that the statement "My suspicion is that this is simply the PR industry at work, hired by companies that sell work attire." is complete unfounded bullshit.
There are many studies that show habits are an integral part of being human. I like the 'elephant and rider' metaphor. You have a finite amount of rider time each day to control your elephant. Every time you make a decision or exercise a bit of self control, you use up some rider. Habits drive the elephant too, but don't cost any rider time. If you are working with your habits, you get the elephant to do what you want for free. If you are working against your habits, it costs even more rider time just to do the basics.
So if you are someone that has a habit of wearing sweatpants and a tshirt all day, forcing yourself into business casual clothes costs rider and reduces your potential productivity for that day.
On the other hand, if your habit is to wear work clothes, then getting up in the morning and dressing as if you are going to an office isn't costing you anything. In fact it's probably putting you in the state of mind that you are 'now at work'.
For me personally, 'getting dressed for work' generally means jeans, comfortable shoes, and a button down shirt. Some days a tshirt. If I'm working from home - even on a Saturday for my own sideprojects (as I did today), I get 'dressed for work'. I wake up, take a shower, shave, have breakfast, make the same coffee as a work day, wear the same clothes as a work day, and then go get shit done. Arguably I would expend less physical energy rolling out of bed and sitting in front of my computer, but I would never ever be in the right state of mind.