Because at the stage of understanding that you are talking about, it's not really the language you are immersing in, it is the culture. Langauge is part of the culture.I don't see any connection between needing immersion in a second language to nail down advanced vocabulary use and "no humans can ever understand each other". The former is just a basic aspect of language learning, the latter is an unrelated overblown non-sequitur.
If you learn a language different enough from your native language(s) it's often the case that some words take a long time to learn the correct usage of, so clearly any short explanation or footnote in a translation is usually not entirely enough and you need immersion. Doesn't mean people can't understand each other.