That is nonsense. You can definitely factor "fun" into a review or impression of a game. From what you're saying, everyone who's played the Forza Horizon 2 demo and came out impressed while having fun is just talking from their ass or something, right? No need to regard any of those impressions then since it's just going to be different for yourself? In that case, why even have forums to discuss games? Just go play every game yourself and see if it's fun for you personally.
Fact of the matter is, a person does not play or enjoy a game if it is not fun to them. If you are unable to give any sort of significance to the fun factor, then having "fun" playing a game is not a legit reason to get a game or even praise a game since as you say, "it is pointless" either way until you experience it. Then there is no reason to even discuss the merits of such game. There are "ideas" and "concepts" of what fun is and it is usually shared among the majority of people. Why do you think so many people enjoy the same game etc.?
Hence, "fun" definitely plays a role in someone's review or impression of the game and this can be measured from a sample size to infer some sort of significant meaning from this data collection.