Not being deeply into signals geekery, I had to lookup SDR. I believe the article is referring to a Software-Defined Radio.
Actually, this is a class D transmitter. Minus the usual output filter to eliminate the higher harmonics. Look at that spectrum chart. The second and third harmonics are huge!
It's quite possible to build something that will turn a square wave into a clean sine wave, and you need that. You can also generate waveforms digitally that have weaker higher harmonics, which means the output filter is simpler. There's something called a class E amplifier, which is like a class D but with an unusual analog tank circuit on the output end to clean things up.[1]
A device like this with no output filter is an annoyance, not a communications device.
[1] https://www.crarc.ampr.org/uploads/Main/Archives/July%20Clas...