You're confused only because you're trying to pick between a false dichotomy. Nkoren basically stipulated that US rocket scientists were too shortsighted/ignorant/stupid to look past their tiny niche to realize that their hyperoptimization for liquid hydrogen was a net negative. Masklinn asserts that it's not true that they were so hyperfocused on liquid hydrogen that they ignored tradeoffs and alternatives, and that in fact they were flying kerosene rockets the entire time as well.

I'm absolutely certain that Masklinn did not assert that the optimal design was used every time or that NKoren's comment was completely lacking any accurate information. Neither of these assertions needs to be made to dispute the central idea that US rocket scientists are too incompetent to even consider alternative fuels or broader design tradeoffs.