No I think it is more about Real Sociedad's attack being weak as they have one of the worst attacks in La Liga. Real Madrid did a simple job here and if Real Sociedad would have had more force in their attacks and more cleverness, then this game could have ended in a different way. The statistics were about equal, but this is Real Madrid eventually. They score the goal they need and then barely put in any effort beyond what is needed to take this win with a single goal. All of the players have in the back of their heads what's coming in the next weeks, which is why some minimalism is going on here.
Toobad for Real Sociedad though because being 1-0 and having to turn this around in the Bernabeu seems unlikely, but still it is only one goal, which may keep their hopes afloat.
Real Sociedad relied on their defense, but against an attack like Real's, it is very difficult to do. After the goal conceded, it was difficult to change strategy, although they had enough time, but they could not do anything, so Real gets an important victory. Ancelotti made a small rotation for this match, but this did not help Sociedad either, although they had several good attacks, Real's defense played this match perfectly.
You do need a goal of course, especially when you are Real Sociedad and you play at home. If you don't score in the first game, the probability that it ends with a draw and you still are well situated for the second game is too low. Real Madrid wanted this one goal and anything but a magical night for Real Sociedad will inevitably lead to losing the second game or at least not winning it, which is still sufficient for Real Madrid.