It sounds plausible that "on demand" instances i.e. search servers are a bottleneck, while bulk offline processing (in the back-end) i.e. PageRank calculations is not such a constraint.

Yes, those offline processing instances could be suspended while dealing with a traffic peak, but peak traffic is still often a challenge, especially now that Google is pushing more and more stuff onto the web servers (Instant).

That might be what he meant.

Disclaimer - I don't work for Google, it's just a guess.