If you only electrically bond (no grounding) the refuel truck to the aircraft, the bonding system will neutralize the static electricity potential difference between the refuel truck and the aircraft. In other words no static spark possible.
Now, if you only electrically ground (no bonding) the refuel truck to earth and only ground the aircraft to earth, there still could be a static electricity potential difference between the truck and aircraft. So in other words, a static spark is possible if you touch the refuel hose nozzle to the aircraft.
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Exactly this.
The earth is a poor conductor, it's just better than nothing. Bonding the airplane to the fuel source is the important thing, grounding to the earth, each or both, is less effective.
To add one more theme to this, as firefighters, we were trained that if we ever had to work around downed, possibly energized, electrical wires at a car accident scene (or direct car occupants to self rescue), all walking steps were to be very short shuffling, definitely not strides. The reason being that the poor conductivity of the ground (in the earth sense) meant that over the distance of a stride, there could be a sufficient voltage difference to be harmful - lower voltage up one leg suddenly getting higher voltage up the other when the step was completed. This could be injurious. Short steps or shuffling would minimize that as much as possible. 'Never a good situation, just make it the least bad it can be.