Since the original article doesn't give a lot of context as to what these characters looks like in context, here is an image of a Japanese book [1] and some playing cards [2] to give context to the segmentation difficulties.
Writing like this is more or less analogous to cursive handwriting in English.
I mean, if you write out "minimum" in cursive, someone who learned English exclusively from printed text might think it was a scribble. But someone familiar with cursive decodes the whole word in a chunk, partly from understanding how latin letters get transformed into cursive, and partly from having seen "minimum" written in cursive before, and maybe also using nearby words for context. There's a roughly similar thing going on here.
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).
That's a good point. My last name, when written in Russian cursive, looks like a series of waves, and is effectively indistinguishable from a child's scribble.
