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To expand upon this:

>> They're measuring it by looking for phase differences in the received L-band (~2GHz) signals

The "L-Band signals" are GNSS signals, for example GPS L1 and L2, which use a carrier wavelength of 1575.42 MHz and 1227.6 MHz, respectively. Both L1 and L2 signals are emitted at the same time, but experience differing levels of delay in the ionosphere during their journey to the receiver. The delay is a function of total electron content (TEC) in the ionosphere and the frequency of the carrier wavelength. Since we already know precisely how carrier frequency affects the ionospheric delay, comparing the delay between L1 and L2 signals allows us to calculate the TEC along the signal path.

Another way to think of it is: we have an equation for signal path delay with two unknowns (TEC, freq). Except, it is only one unknown (TEC). Use two signals to solve simultaneously for this unknown. Use additional signals (like L5) to reduce your error and check your variance.