Normal Googlebot IP addresses resolve to names like crawl-blah.googlebot.com. These ones don't. You could use whois to detect them, but it's a lot slower than DNS. I guess that Google expects the SEO guys not to be smart enough to use something like pwhois to quickly check that the IP's on a Google AS (likely).

There are some google ip addresses. Who knows how many google but registered in another company's name IP addresses they're doing this sort of thing from