Imposed or accepted it doesn't really matter, similarly the person I responded to specifically referred to anarchism as nonheirarchical. What you described is communism or socialism, not anarchism.

If we "never see people deliberately leaving small egalitarian societies for larger hierarchical societies," heirarchical structure l society wouldn't exist. Similarly the hadza (an at least claimed non heirarchical/egalitarian society) constantly lose population to "modern" society.

That said, agriculture is a trap in the sense that once we adopted it, we could and can never go back for a set of reasons that should be obvious. The task then is to most nearly recreate the system we lived in for 99.9 percent of our existence as a species, while still accounting for the fact that we live under a new set of parameters and can never go back to those that existed before.

That's communism or socialism, not anarchy.

As I understand it, this is the puzzle that some forms of anarchy set out to solve.

It's not a puzzle though, a puzzle eventually fits together but anarchy simply doesn't.