(which incidentally includes a list of some phrases which are often miscredited to him, eg "All that glisters is not gold" and "Et tu, Brute?"

By that do you mean that he did not coin the phrases in question? Or were you referring to the misquotations, "Et tu, Brute? instead of "Et tu, Brute!" That's a bit like the old "Elementary, my dear Watson", "play it again, Sam" malaquotes Looking at the quote from the MoV, he seems to be very up front about the fact that the saying is not original: "often have you heard that told." As for the other, well I guess we would have to assume that eyewitnesses to the event in question remembered what the "pompous ass" said.

The Merchant of Venice
Act 2, Scene 7
MOROCCO O hell! what have we here?
A carrion Death, within whose empty eye
There is a written scroll! I'll read the writing.

[Reads]

All that glitters is not gold;
Often have you heard that told:

Julius Caesar
Act 3, Scene 1

CAESAR Et tu, Brute! Then fall, Caesar.