FSD vs navigate on autopilot is a pretty small technical detail on the highway. Once you turn on FSD beta, from the driver's perspective FSD and autopilot are the same thing - turned on with the same input (double tap on the right stalk), with the same stuff shown on the display.

Navigate on autopilot can change lanes on the highway and follow highway exits without user intervention.

IMO when someone says they're using "FSD" they mean they're subscribed/paid for FSD, have the FSD beta turned on, and are letting the car drive itself (whether that's FSD on city streets, or navigate on autopilot on the highway.)