activation in this context means spawning the unit. So the unit wont be placed until the trigger condition for late activation is met.
what you could do to make it work is uncheck late activation, and have a hold command in the advanced waypoint options for the fist waypoint of the unit. The hold command you can give a stop condition, such as time, or a flag. When this is met, the hold action will be stopped, and the unit will start following its waypoints.
or you place the unit and pause/resume it with group ai on/off triggers