firstly, "the referee may award a penalty try if, in his opinion, a try would have been scored but for unfair play of the defending team". The rules don't say anything about being 100% guaranteed. if you're 99.9% certain then you award the fucking try Wally.
secondly, what was Gus going on about with the try/penalty try business? I don't think Shane Hayne thought Lyon had legitimately grounded it. He was awarded a try because he thought it was a penalty try and was asking the VR to confirm.