Look at the other 2 lines of the caption.
The line that says "Answer is: PHP5" was presumably added later.
It was there when this news.yc item had 0 comments. Also notice in the flickr comments the answer was given 6 weeks ago.
Yeah, obviously. The OP's point is that attributing the drop in CPU usage to PHP5 (which was the answer to the original Flickr poster's question) is non-obvious (or it is a plausible candidate among a bunch of possibilities).
If he wrote "how might one guess" instead of "how did you know" I'd say you were right. Occam's razor suggests it was a question directed at vaskel as to how he chose to title this item.
Is this supposed to be obvious? Otherwise, how did you know?