True, what Socrates is purported to have said in the Phaedrus could well be a concoction of Plato's -- we'll never know for sure.

However we do know at least by virtue of the existence of the Phadrus that Plato himself did not subscribe to the position attributed to Socrates in the Phaedrus.

Socrates being probably the most quoted person of all time who never wrote anything (giving Homer the benefit of doubt since he did leave a couple of epic poems).