In C++ most operators could end up invoking an actual, user-defined function at runtime. But, again, nobody is claiming that they therefore behave like functions.

I get the logic--if you squint really hard you can analogize sizeof to a function. It just doesn't work, as evidenced by his own example. C isn't LISP. It's C. It has a unique grammar with distinct classes of constructs. sizeof is a unary operator, parses exactly like every other unary operator (tokenization conflict with identifiers notwithstanding), which is quite different from how function calls are parsed, particularly wrt parentheses. sizeof has some unique characteristics, but every operator has unique characteristics; that's why they're operators, as opposed to some functional languages that try to subsume everything into function-like syntax.