After the extermination of Melos they could credibly say they were less responsible for the actions of the polis.
And had a higher chance of deflecting the inevitable revenge on to the non idiotes Athenians.
For one thing, wouldn’t everyone claim they were against their old polis? How would the invaders have any idea who was an idiote?
I just don’t believe it’s at all easy to avoid the fate of your nation , and I especially doubt that the politically ignorant have a better chance of avoiding that fate than the well informed.
The counter extermination was only 5% of Athens total population, or so historians say, so it seems like a lot of nuance was shown.
That fact alone doesn't demonstrate nuance. It's possible that 5% of the population was innocent and treated as scapegoats, or chosen randomly, or that anyone high profile regardless of guilt was chosen to die.
Unless there's data on who was actually innocent or guilty, the mere fact that extermination was selective doesn't mean it was in any way accurate.
Of course nobody can prove either way beyond a reasonable doubt for something that happened so long ago.