Hopefully you are young enough to have been spared the indignity of witnessing a plague of fluorescent legwarmers and sweatbands escape the aerobics gyms of the 1980s. :)
A lot of modern casual and even business-casual fashion is (or was) sports wear.
Polo shirts? Explicitly designed for tennis—it's the reason the design exists. Pure sportswear.
Oxford-cloth button-downs? Sports wear. That's why it's got the buttons to keep the collar down, and why the cloth is so heavy-weight and rugged.
Rugby shirts? Jerseys? Of course.
Sneakers, of course. Also, many other shoe designs, from equestrian and hunting sports.
What's not sports wear is often former underwear. T-shirts and A-shirts (thought the latter remain very informal, at least) were/are underwear. Arguably leggings are underwear that (somehow—this trend remains baffling to me, but on the other hand, I mean, thank god for it, obviously) became sports wear and then became everyday casual wear.
I'll sometimes do this sort of thing when I travel. Lot easier to travel light when you don't need a separate pair of running shoes.
but the thesis was basically: sports wear has often influenced mainstream fashion since even like 100 years ago.
but in each generation, people find it initially shocking to see so many people wearing what is marketed and made for sportswear for leisure, or even seeing it spill into the workplace. i think the article was focusing on leggings when they started creeping into non-sports-related settings.
one of the craziest counter-examples i've seen, from the anecdote you share about your partner, was when i saw a man dressed to the nines in a really sharp/ smart business suit, looking like he had a great haircut. but for his kicks, he was rocking Nike Vaporfly 2's, which are known to only have so much life in their foam specialized for distance racing. i personally didn't think it looked at all aesthetically pleasing, but because they were relatively expensive for running shoes, i'm guessing that the man decided to pair them with his high fashion look.