That's not what he means. Hes talking about making a new friend at a mutual friends party, lets call him Joe. You look at Joe's profile, and all you see on Joe's profile are photos of him drinking.
Meanwhile, Joe has been sharing a ton of geocaching and hiking posts with his "hiking" circle, but you don't even see it since he didn't know that you liked hiking, he only knows you from a party. His alternate interests are hidden from you because you weren't in the right circle on his end, completely out of your control.
I'm not on Google+ so I can't really say for sure, but I understood Sparks to be more like a global stream that you subscribed to. I didn't understand that Joe would write his hiking posts into the Hiking Spark unless he was sharing something he thought strangers would be interested in.
It still demonstrates the issue with circles based on what content your friends would be interested in. Circles seem to make perfect sense to me for security (you don't want to see your mom posting photos of you partying maybe), but for interest based valuations it seems like you pretty much always want it to be public (at least public on your profile page, even if you don't particularly want it to show up in other peoples 'feeds')
Meanwhile, Joe has been sharing a ton of geocaching and hiking posts with his "hiking" circle, but you don't even see it since he didn't know that you liked hiking, he only knows you from a party. His alternate interests are hidden from you because you weren't in the right circle on his end, completely out of your control.