I think the point is that a user wouldn't know they're overriding the design. When a user sets a uniform font, they're opting to not see whatever font the site

I think the point is that a user wouldn't know they're overriding the design. When a user sets a uniform font, they're opting to not see whatever font the site was designed for, but it's not at all obvious to them that they'd be messing with icons.