No, it's not actually. You're talking about two different things and making pretend they're one.
Postmodernism is a specific philosophy.
The long march is a specific strategy by a specific postmodernist thinker to spread his ideas through society. Subversion isn't a technique specific to postmodernists or communists. Capitalists have no qualms about using it either.
If you really think postmodernism is so terrible come up with better ideas and make better arguments.
The long march was a strategy applauded by the originators of post-modern thought, of the now-infamous "Frankenfurt school". It was generally acknowledged, atleast at the time, as the way to go forward with the Marxist revolution.
Subversion isn't limited to post-modernists but no one has put together a more comprehensive strategy on how to gradually subvert a population culturally in the modern-day context, than post-modernists.
The reason why the subversive efforts of post-modernists are met with claims of conspiracy more often, is because there is obviously a lot more academic intellect and conscious planning behind the attempts to influence, compared to those of a Donald Trump, or the Fox News. Crude strongman bullshit simply does not compare to comprehensive, critically evaluated theories put together by intellectuals of various sort.
The reality is that whenever people feel their intelligence is being belittled, and that they are being manipulated to think a certain way, they are going to lash out. Not necessarily because all of the ideas themselves are bad, but because they are being presented in a dishonest and indirect manner. Anything that is being disproportionately forced on people, is going to meet a backlash, that is only natural.
I've never said that all of post-modernism is terrible, but I think that there are many flawed patterns of thought behind it. One of the consistent pests of Marxist-influenced thought is the idea that a "revolution" of sorts must ensue. That's something that even ol' Karl Marx would've liked to have had back, if he were able to look back from today's perspective. It leads to a generation of youth who become hysterical about requiring immediate change, instead of being able to witness human history as a gradual, stable progress, usually only interrupted and delayed by these sorts of "revolutions".