I have no idea about the theory behind it, but i can say with absolute certainty that every self-described post-leftist i have ever come across was an anticommunist wrecker with universally awful takes on literally any subject we unfortunately went over during our way too long and thoroughly horrible encounters.
I need to add that the sample size for that is absolutely tiny, this is fortunately an extremely fringe micro label hardly anybody uses, but i am extremely burnt on the handful of people who do.
The absence of theory behind Post-Leftism is the theory behind it.