There are differences between the systems of a private capital and of a nation (or for that matter, the entire globe, which has never been planned centrally; incidentally this explains black markets).
The difference in goods is a mere quantitative difference. (Amazon sells literally everything.) Still, the amount and diversity of crap managed by Amazon is much greater than that of North Korea (comparing net sales vs. GDP). "It seems that there is simply too much in an economy for one entity to wholly plan" has to be tongue-in-cheek if it's not simply a mindless mantra. It's a staggering ideological contradiction whose poles the Hacker News crowd must oscillate between. On one hand, we're on the cusp of generalized artificial intelligence, and on the other we can't figure out the linear algebra needed to maintain a input-output table that changes over time.
But the difference in competition is a qualitative difference. The laws that determine the "competition" between teams at Amazon are different (opposite, even) than the laws of competition between capitalist firms. This is an empirical question. We can certainly imagine Amazons teams undercutting each other's "prices" in order to maximize private "profit," leading to a centralization and concentration of "capital" in the hands of a "monopolistic" team, but this simply does not happen in practice. The categories of intra-firm operation are totally different than those of inter-firm operation.
