The fact that he says he did. The fact that he is credited on the record sleeve with doing so. And here's an interview where he says he did: John Taylor Talks Duran Duran, Early Influences and that "Rio" bass line
It's amazing how he apparently wasn't capable of playing those basslines on the records, yet was able to develop the skill to do so when playing the songs live immediately after the records were released (and likely beforehand). But you know, if someone's brother's hairdresser's best mate's football coach said Taylor didn't play them, then it MUST be true (in spite of any evidence to the contrary).
Why don't YOU present actual evidence that he didn't? You're the one who's claiming Edwards played the bass on those tracks - so provide evidence he was the one who did.
So in your world, no one ever lies, no one mimes to backing tracks and the music industry is a paragon of virtue...
You believe because you want to believe, the truth has got nothing to do with it. Like I said, incredibly naïve...