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The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
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Note that it does not say the "surface speed of the wheels' outer diameter", it says the "speed of the wheels".
If the plane is at rest, the wheels are too - going 0mph, and the conveyor belt is not moving either.
If the plane is going 5mph, the wheels are also going 5mph (right along with the plane they are attached to no matter how fast they are spinning). The conveyor belt is going -5mph (the "-" meaning the opposite direction). If the plane had a speedo connected to the wheels it would falsely read 10mph. You would use 10mph to calculate the RPM of the wheels given the diameter.
If the take-off speed of the plane is 150mph, the wheels are also going 150mph (moving right along with the plane). The conveyor belt is going -150mph. If the plane had a speedo connected to the wheels it would falsely read 300mph. You would use 300mph to calculate the RPM of the wheels given the diameter.
Whenever you talk about the speed of one thing versus another, like say a wheel versus a conveyor belt (note it says "belt" not roller nor the whole conveyor), you have to use the same units of measure. I have designed and spec'd 100's of conveyors. When you talk about conveyor belt speed the unit of measure is m/sec, or feet/sec - in other words linear speed which of course mph is. Rotational speed is not the same thing - an object can be spinning at 1000 rpm and still be going 0m/sec (like a house fan for example). So if you are going to say that a conveyor belt is going "to exactly match the speed" as a wheel it has to be in linear speed units.