You have to be able to guarantee he was going to score though. The issue is with the rule not the ruling.
The thing about penalty tries, is that phrases like '100%' and 'guarantee' are a complete fabrication by the fanbase*
The actual rule, copied from the rule book here
the Referee may award a penalty try if, in his opinion, a try would have been scored but for the unfair play of the defending team
All it takes his for the referee to think a try would have been scored.
So in that sense the decision to award (or not award) a penalty try can never be incorrect, because who is to dictate what the referee's opinion is
But you just know at some point in the next couple weeks, as a "square up" for recent decisions, someone will get sent off for tripping, and someone will get a dodgy penalty try,
* As a side note, one of the biggest causes of people's negative opinion of referees, is that they judge their decisions against what they think the rules are, or what they want the rules to be. Not on what the rules actually are. Phil Gould is a massive offender here, and I am so so so so so so so so so happy that I can watch Fox now and never have to listen to him again.