He is muting the string when he does it. If that's what you're getting then you're doing it right; Guthrie never uses the left hand slap to produce real notes.
R.I.P. My Signature. Lost to us in the great Signature Massacre of 2014.

Quote by Master Foo
“A man who mistakes secrets for knowledge is like a man who, seeking light, hugs a candle so closely that he smothers it and burns his hand.”