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.