This makes no sense. He is supposed to make his judgement from the demo on whether he would want to play the full version of Journey. Its the whole point of a demo. If he didn't like the demo then what makes you think he'd like the full game? If he thinks it is overrated from the demo, he thinks it is overrated.
Yes that's the point of a demo, to decide whether or not you are interested enough to even play the full game. Then after playing the full game you can form a final opinion of it.
It's just like the movie trailer analogy people have tossed around, after watching a trailer it would be fair of me to say "that movie looks dumb" as opposed to "that movie is dumb". People were just saying that it was unfair of him to base his opinion of the full game off the demo even though it is fair to base your decision on if you even want to play the full game off your time with the demo.
Obviously it's not out of the ordinary to base your thoughts on a game solely on the demo but at the same time as other have pointed out demos are not full/final games.
I can't tell you how many times I've played a demo and just not gotten into it only to play the full game later and have a blast, but to say a game is overrated after playing the demo (especially in this instance) is a bit ridiculous.