They mean not leftists enough, its a way to absolve themselves of any responsibility of the situation at hand, if only they were even more leftist.

You will note of course that the rules and regulations around how to implement these things are always 'bipartisan' and meant to stir up as little backlash as possible. There's many opportunities to install bottlenecks and roadblocks in systems that end up shaping the entire way they function. To some extent it is progressives anticipating conservative backlash but there's plenty of lines in plenty of laws written into and insisted on by conservatives that have wide-ranging consequences.

Reductionist perspectives on how politics works always end up at this "one side is dominating the other" kind of narrative but it's never actually that way, both sides still have a lot of influence on the various specifics of the outcome.