Maybe people talk like that, but in my opinion Barca has a much stronger team than Real, Ansu Fati provides great games, Griezman has yet to show what he knows and with Messi it is an amazing trio with having Coutinho as an assistant, Real struggled a lot previous the game may be a good environment and defense, but they are hampered by the attack.
Much stronger?! I disagree if you state "much stronger", how can? Real Madrid also has good players and they are possible to win el Classico. For me, Barca only has Messi in the front line as the most effective striker after Suarez leaves. While Real Madrid has no big difference in their main players. Both Barca and Real Madrid have the same chance to win in el Classico. Don't underestimate Real Madrid, they the winner last season, right? 
I wouldn't say stronger but maybe equal.
It is the same when people say that Real Madrid is better ignoring what Messi has done all those years.
In the end, El Classico is a game that everything can happen in 90' minutes. A team might be in a good day or not. We should not underestimate neither Real nor Barca.