I'm not a network engineer, but it seems like the kind of thing that might be very hard to detect when you're already inside or near to the google.com domain. Or maybe CloudFlare just got there first.
I don't think it's necessary to call BS on Cloudflare without any kind of evidence at all.
This is basically correct. BGP is weird. The addresses for one of Google's many datacenters were routed incorrectly for packets coming from some subset of IP space. Unless Google is running active ping tests to that subset of IP space, the way they would normally detect it is for someone to call and complain.
In this case, the author decided to take a shortcut and call the owner of the "problem peer" directly.
Although only a vanishingly small percentage of Google users can call and complain. Blog or tweet or post to HN and hope Matt Cutts sees it and notifies the right team, maybe.
A team of Googlers could have been working on this in parallel to Tom. I'm guessing that a sudden drop of queries like that would cause people at Google to start digging into what happened. I don't know either way, because network ops and BGP is pretty far from my area (search quality).
A common way to notice things like this is to subscribe to a service like Renesys or Cyclops (http://cyclops.cs.ucla.edu/) that will alert you if it sees your subnets being announced by a different AS.
I think it's good to qualify your opinion with your level of expertise. There's no rule that says HN should only be for discussion by experts (hopefully there never will be), and if you don't know something for sure it's best to say so that others don't take your word as gospel. That said, I'm no expert ;).
I don't think it's necessary to call BS on Cloudflare without any kind of evidence at all.