Trouble is, people then get used to macOS’s font rendering, so that (a) they don’t want to turn it off, because it’s different from what they’re used to, and (b) start designing for it. I’m convinced this is a large part of the reason why people deploy so many websites with body text at weight 300 rather than 400, because macOS makes that tolerable because it makes it bolder. Meanwhile people using other operating systems that obey what the font said are left with unpleasantly thin text.
I have a vague feeling that Macs also have somewhat different gamma treatment from other platforms, which would be likely to contribute to thickness perception.
In Leopard that font smoothing preference wasn't a checkbox, but a drop-down menu specifying how much font smoothing. If you select "Strong" you get a result that looks even bolder than normal. Even when a website is using an extraordinarily thin font, this setting makes it readable.
Apple replaced the drop down menu with a checkbox in Snow Leopard, but the underlying functionality wasn't removed until Catalina. Naturally I was very disappointed after my upgrade to Catalina because all text looked too skinny to me.
If you’re interested about the general topic, though, give https://mbtype.com/pdf/equity-type-specimen.pdf#page=5 a read!