Not the maximum frequency, but the maximum bandwidth. You can sample 1 MHz band at 100 MHz central frequency with 2 MHz ADC and recover the signal (provided your filters are ideal of close). This is called undersampling, and this technique is often used in practice.
This note is unrelated to Hack_RF, I don't know what they are doing there.
In the bottom left box, you can see that they are interested in the 200 kHz bandwidth, so sampling it at 20 MHz is not a problem.