But that's the thing, according to the exact definition, he should be preventing De Gea from playing the ball by clearly obstructing his vision or challenging him. Influencing the opponent by his mere presence is not included in the definition. Intuitively, I'm sure we'd all say him being there is interfering but I can find nothing to support that in the actual definitions. I guess it all sounds a bit silly and I'm looking at this from a purely academical point of view, but I can't help it, that shit interests me.
Having said all that, without over analyzing it, it's the sort of decision you can understand while at the same time understanding why the other team would be furious. Just one of those things.