[glandium]

That is absolutely true, but, like cursive handwriting in English, there are cases where it's all gibberish, even to natives. See, for example, how doctors handwriting is barely readable. With effort, you can make up the words you are familiar with. But not the others.

Now, imagine some text in medieval english in doctors handwriting. That would be basically undecipherable for most people.

The same is true about many Edo period (and older) writings, which are written as "scribbles", and use old vocabulary/grammar that most people don't know. They are essentially unreadable to most Japanese, except the trained ones (e.g. historians).