It says…

Fortunately, sneaking in an executable, an image file, or an application extension isn’t possible.

Then says…

While the title refers to “arbitrary data,” users can hide whatever they want within Unicode characters, though this seems limited to text.

B64 is just text, and pretty much anything can be converted to B64. This can absolutely be used to exfil more than just text-only data (say, a db or media). Therefore, this isn’t the non-issue they make it out to be, and should be seen as what it is.