Ha ha you're right. It's just one of the regular ones I see daily and when I pull people up on it they look bemused as in the industry you see it in publications. It's a little bit like sayings that are regularly misquoted but have become the norm and as such change the meaning.
Jack of all...
Of course you're wrong as a font is a type face whereas fount is a spring, fountain or tap such as beer fount. I have corrected so many people in my job of the correct meaning.