I doubt those very people would confuse the two when presented with both next to each other: / \ / \. The issue is, they're not characters used day-to-day so few people have made the association that the slash is the one going this way / and not the one going the other way \. They may not even be aware that both exist, and just pick the first slash-like symbol they see on their keyboards without looking further.
I wonder if it's dyslexia-adjacent. Dyslexic people famously have particular difficulty distinguishing rotated and reflected letterforms.
Could be. The frequency is such that it could be dyslexics. It's not all the time, but it's a steady rate of incidence.
I think in the 90's it was just people repeating a pattern they learned from Windows/DOS.
It used to grate on my nerves to hear people say, e.g. "H T T P colon backslash backslash yahoo dot com".
But I think they always typed forward slash, like they knew the correct slash to use based on the context, but somehow always spoke it in DOSish.
It’s some sort of mental glitch that a number of people fall into and I have absolutely no idea why.