Surely by virtue of arriving at the finish a number of minutes ahead of the leaders, he did a bit more work than he would have had he sat in the bunch. It also makes me chuckle that on an up and down day people seem to give drafting magical powers, like Yates barely had to pedal yesterday.
IIRC,drafting gets increasingly beneficial from around 12mph (20kph), saving up to ~33% of the Watts being put out by the rider(s) at the front of the group.
The GC group were surging throughout most of the stage, while most breakaways I've watched tend to keep a fairly constant effort once the breakaway is established.
Yates was playing the role that Wout did for him on the final stage of the Giro, up the road ready to help from the top of the penultimate climb, but the GC group failed to catch the breakaway.