The FCC Chairman specifically threatened to pull ABC broadcasting licenses if they didn't punish Kimmel. That isn't circumstantial at all. That's a smoking gun.
The FCC chair, in the unlikely circumstance that that charges for violating the constitution are bought and a conviction occurs, can be readily replaced with another of the same ilk. Changing nothing about the circumstances that find the US with an administration blatantly willing and prepared to go beyond the constitution.
The FCC chair isn't the cat that needs to be belled.
Why did anyone have to ask him? He spoke in his capacity as a government official, and he has the power to do what he threatened. That's sufficient to say "the government is suppressing free speech".