Modern neuroscience knows quite a lot about what is happening inside neurons. Is it "properly" understood? I don't know what you mean by "properly", so I can't answer that. If an acceptable rephrasing would be "... inside a human neuron is still not completely understood" the I would agree. But just because we don't know everything does not imply that we don't know anything. And keep in mind the qualifier in the part you replied to:

at least directionally.

Taking that qualifier to heart, I'd have to say that I agree with @rayiner. We know the broad brush strokes, even if some details are missing.