When I saw the title, I thought this was going to be a security issue... like that they figured out how to identify someone's location based on unique power fluctuations from sun/clouds somehow leaking into a detectable signal in someone's online communications.
Why would an alien race bother with solar panels? Surely they would have mastered fusion (and beyond) rather than littering their planet’s surface with solar panels.
(That is, unless they have Earth-like politicians granting tax incentives, or if alien hipsters install them because they are “retro”.)
Well the paper shows that looking for signatures of photovoltaics is an impractical strategy if not impossible.
The only other games in town, if we're talking huge amounts of power, and ignoring the speculative "beyond", are fission and chemical combustion. Unlike PVs these can be expected to alter the planet's atmosphere distinctively, and that is the thing we can actually observe during transit events, in contrast to panels (and dams, wind turbines, geothermal etc) down on the surface.
Apparently it's worth bothering just to remain inconspicuous. Aliens are clearly doing everything possible just to hide themselves from Earth.
Given where we are now—reproducible demonstrations of net gain, using AI to control confinement—it’s reasonable to think we will see some full-scale implementation in our lifetime. Its a virtual certainty humanity will achieve practical fusion before the year 3000, unless of course we use fusion bombs to destroy ourselves.
Technosignatures are pure speculation. There's no reason to believe aliens exist or utilize any particular technology. But if you assume they do, you have some handles you can grab to try and work the problem.
I guess the answer to your question is, "why not both?" Look for solar panels, look for fusion reactors, look for whatever you can think of. It's sort of like diversification, the wider your net the less you are speculating. Taken to the extreme where we cover all possible technologies (clearly impossible but bear with me), our speculation becomes "aliens exist and use technology."
It is as non-sensical like searching for a radio using SETI. Nobody sane would use an all directional EM transmitter with power of GW+, when trend on Earth is to use wider channel with lower power so you don't disrupt other channels few kilo meters away.
And for longer distances you would not be wasting EM power in all directions, but using highly directional beam of EM energy towards target or relay satellite.
