Smart RTH is when the button is pressed, not when the battery gets low:
I think the manual is a little misleading here.
When you think about it Failsafe RTH and Low battery RTH are really just submodes of the overall Smart RTH system that can as it happens be manually triggered with the button.
If you take the time to look in the menus in the DJIGo4 app, you will see there are several smart return RTH settings that are not related to triggering RTH with the button.
Under battery there is a setting called
Smart Return Home which enables the aircraft to RTH on Low Battery. ie Low battery RTH is a Smart RTH feature. This contradicts the interpretation that they are 3 separate modes.
Under signal loss behaviour there are options to RTH, hover, or Land. The default RTH option being the Failsafe RTH submode of the Smart RTH system.
Finally under Visual Navigation Settings > Advanced Vision Settings you will see there is another setting, Turn on
Smart return home. This enabes the check for obstacle during the RTH flight phase even if OA is disabled.
I have not tested it but I am quite sure the
Smart RTH obstacle avoidance will work if the RTH trigger is Failsafe or Low battery. It is not limited to working when you press the button.
Now if pressing the physical button (or onscreen button) to manually trigger RTH was really what DJI meant is exclusively is Smart RTH, then logically disabling the Smart RTH in the menus would disable the button. We all know this is not the case.
So in the context of OP & the message
NFZ NEAR Smart RTH and intelligent flight functions will be limited
I think this simply means that once a RTH is in effect, the intelligent flight functions within Smart RTH system, such as ascending or flying around obstacles will be curtailed by the adjacent NFZ. Furthermore if flight path had managed to take the Mavic to a position that placed the NFZ between it and the home point, the Low battery
Smart RTH function may not be allowed to transgress the NFZ to get home. So the aircraft may not be able to complete the RTH.