The cessna take off speed is 100Km/h, does that mean that refueling truck had to go around that same speed, or maybe a little slower since the airplane did not have to rise?
Am not a pilot, but why would they need takeoff speed, rather than to just stay above the minimum speed before stalling? That $V_s0$ speed is more like 65Km/h.
Also important to note that's airspeed, not ground speed. If they're driving/flying into a headwind, the ground speed (the speed the truck cares about) is even lower.
I build camera gear for 172s and fly (as passenger, not pilot) probably 20-30 days per year, sometimes much more, and I get nervous at <60kts at low altitude.
40kts IAS is I guess around the full flaps stall speed but you sure don't want to be doing that intentionally. At least not with me in the plane please.
Or did they have a modified wing?