The advice, while based on the fact that 2.4 GHz WiFi behaves like radar (in fact, their frequencies are similar), does not mention that there are other, easier ways for a local physical attacker to detect the actual keystrokes being entered. An attacker close enough to pull this off could use the simpler, easier, and more accurate technique called acoustic keyboard eavesdropping. This attack involves using a very sensitive microphone to pick up the unique sounds made by each key on a keyboard as it is hit and using that signal to reconstruct, with high accuracy, what keys were actually pressed.
If they could not do that, then they could use Van Eck phreaking, a technique often referred to by the codename TEMPEST, to eavesdrop on the electromagnetic signals emanating from the keyboard, usually signals coming from the microcontroller or cable from many tens of meters away. This technique can be done on both PS2 and USB keyboards. There are keyboards immune to this, but they are expensive and specialty devices that are not easy for a consumer to obtain.
However, it's important to point out that 2.4 GHz WiFi, even when used as radar, has very poor spacial resolution. After all, the wavelength is 12.5 cm. This means it would take a lot of clever tricks to use it to analyze something as small as a finger. Even if that could not be pulled off, the simple fact is that an attacker near enough to try such an attack has better ways to get your keystrokes.
