Way too much.
If that quote is true, he said that Juve is his reality, the biggest team in Italy for which he'll play with all his heart. And then he added that Real is just a dream of his. How much better would people feel if he kept it to himself? He was still gonna dream about Real, wasn't he? Just that you wouldn't have known about it.
If I was a football player and Manchester United wanted to sign me but Juve didn't show interest, I'd give my best for United, but I will still dream about Juve, even if Juve are at the time a smaller and less successful team.
We already had many self proclaimed "other big team" fans in Juve who played their best football exactly for Juventus.
What Isla said is just naive and earns him bad reputation among Juve fans, but essentially it changes nothing.