In the macOS User Guide, it says "Font smoothing reduces jagged edges for some fonts. When text smoothing (or “antialiasing”) is on, smaller fonts may be harder

In the macOS User Guide, it says "Font smoothing reduces jagged edges for some fonts. When text smoothing (or “antialiasing”) is on, smaller fonts may be harder to read." That seems misleading, though, because there is clearly some amount of antialiasing happening regardless of which option is chosen. I think you're probably right. To be honest, everything in macOS seems to be designed with that switch on, so turning it off makes fonts in many areas look noticeably worse.