It’s a terrible idea, having been a biker in socal. You’d think ventilated fabric, even a regular t-shirt with air moving on you from your forward motion would cool you…
It doesn’t. You’re actually adding heat (thermal energy) to your body faster than it can be removed by your sweat glands, the glands cannot output water fast enough to match the thermal energy being injected into you as the hot air passes over your body. You actually need to insulate yourself with a wind blocking jacket, at which point fuck that, stop riding, and wait a few hours.
It’s a very odd sensation, but you understand it almost without explanation if you’ve tried it… So what were these guys thinking, ignoring their body’s alarm bells?