Most digital scopes have around 5-10 times faster sampling than bandwidth. The one on pic is 250Msps.
That's more than good enough for the purpose of checking interference
But he tries to quantify this interference.
Anyway Animats's comment is the one that points IMHO to the most likely cause of the observed waveforms.
https://news.ycombinator.com/item?id=47931024
PS Now that I’ve taken a closer look, this is even sillier than I first thought.
He’s hunting for 50 MHz ghost signals by connecting his PCB to a breadboard using (crappy) wires that are at least 10 cm long. And he’s connecting the scope probe to the breadboard (or those breadboard wires).
And if I’m not mistaken, he doesn’t even bother to connect the ground lead of the probe.