I interpret this as meaning that Trump doesn’t remember every single conversation he’s ever had and didn’t specifically remember the conversation with Billy Bush, but upon hearing the tape, believed it was something he might have said, in a boastful tongue in cheek sort of way, so he accepted it as true and went from there. Maybe since then he’s started to have doubts about its authenticity and whether or not the audio was modified in some way.
I interpret it as meaning someone's spreading BS about what Trump has been saying about the tape.