Pooled mining takes mostly care of this on the bigger pools, at every difficulty increase the mining pools see an increase in hashing power. Solo miners typically dont get more hashing power for every difficulty increase.
The poo doesn't get bigger due to magical pixie dust. It gets bigger due to hashpower being added, same way solomining gets biggerr.
So my conclusion, solomining on a realistic timeframe will on average produce less BTC for miners than pooled mining and thats why I agree with the wiki's statement about this.
By using the words "on average" you make your statement completely incorrect as many have pointed out here.
It's fine to say that there is increased risk of higher or lower payouts, and the low payouts matter more to you because they'll put you out of business (or the higher means less because you somehow don't need more coin(??))... and thus on average solo has lower utility for you, but that lower utility != less btc _on average_.