> It really was years of detective work

Doesn't seem that complicated from the article though. They say that they immediately knew it was clock based due to the strict interval. With REA etc. it might become easier to proof, but I guess it's a measurement problem that only appears in large networks?

Can someone with more understanding explain why this was apparently so difficult to find out?

There are many millions of devices connected that draw power, and increasingly so, supply power.

The effect is apparently small enough that they had to use Fourier analysis over many minutes to see it in the noise.

If it were not a substantial research effort spanning years, it would not get funded. This motivates academic researchers to ensure that every topic is explored thoroughly and at great length.