Cadence sensor to measure speed?

Cadence measures the rpm of the pedals/cranks, not the wheel/bike speed.

Remember that cadence is the measure of the number of times an event happens. When running the cadence is the number of steps you take, in cycling it is the number of times you rotate the crank.

A cadence sensor will measure the number of time you spin the crank

the speed sensor will measure the distance traveled by taking the number of times your wheel rotated and that is used to calculate the speed/distance using a known wheel circumference.

The edge/fenix will read your cadence sensor and simply display RPM. It will not use that input to calculate seed/distance because, you know, gearing. Your gear ration can change while your wheel circumference does not.

no one single sensor can perform both actions at the same time

Before the fist smartass wakes up, yes, you can buy a speed/cadence sensor but these still take in two different inputs, usually using a crank and a wheel magnet, to get speed and cadence

Some magnet-less cadence sensors can be converted to speed and vice versa with a button push, or removing and re-inserting the battery.

You will need to have purchased a dual purpose sensor though, I have no idea if the Wahoo is dual purpose.

But if you want speed you need to run in speed mode (if dual use) or buy a speed sensor if its not.