It means he lacks the ability to reinvent his comedy to fit the times. Instead he just complains.

We all need to continue to reinvent ourselves over time. Things change. We need to retool for the times ahead. Reality is Seinfeld doesn’t need to because he already made his money.

So instead of him getting back on the horse he just sits back and complains like the out of touch old turd that he is. He sucks but just can’t accept it or admit it. It’s easier to whine.

He is a has-been and never will be again.

"Reactionary" means regressive conservative/anti-progressive. It originates from, as much political terminology on the regressive vs. progressive divide does (including the terms "left", "right", and "conservative" themselves), the French revolution, where people who favored opposing the revolution (i.e. reacting to the revolution) were called "réactionnaires" in French.

Here's the first known usage of the word in English, from a 1799 English translation of Lazare Carnot's letter on the Conspiracy of the 18th Fructador.

Reactionary can be seen as a more extreme version of conservatism.
Conservatives want to keep things as they are and oppose change.
Reactionaries want to turn back to some previous, supposedly better state.