>And whoever wrote the original sentence can always come back and ask “but did you really really understand them”?

Asking questions that are very difficult to answer precisely and well, and then trying to do so, is kind of the whole deal with philosophy. How /do/ you know the things that you think you know? You can roll your eyes at it and choose not to investigate further, or you can try to get super granular about sussing out an answer. Philosophers generally try to do the latter.

If your answer to "what does it mean to know things" or "how do we actually use words to navigate meaning" or "how well can we understand other living things" is "I don't care, that's nerd shit" then I highly recommend that you don't read Wittgenstein, because you're gonna get real bored real fast.