Not necessarily, there are valid “characters” that are not rendered in most text parsers, or are rendered as whitespace. A great example of this is the byte-order-mark (BOM). You can embed a BOM code in the whitespaces of a text string and it looks exactly the same to a human as one without.
I assume that’s what they’re talking about here
Here’s an example of a steganography encoding technique using this method. 330k.github.io/…/unicode_steganography.html