Completely agree. This statement is immediately disproven by the authors following points. Eg pointing out that the supreme court, and other authoritative bodies exclusively use serif fonts...

Of course there is no "a priori", the general public doesn't know what a letter is "a priori" until they are taught. At the same time they are taught which fonts are formal and authoritative and which are not.

Everyone knows Comic Sans is not appropriate for a legal brief. No matter if that is "a priori" or not.