Based on a few tests, I think he's just doing DNS queries and assuming any NXDOMAIN response means a domain is unregistered, which is not always the case. Sure, users figure out the domain isn't available once they try to register it, but by then you've already dashed their hopes.
It's fine to use DNS queries as part of the checking process, but if you get a NXDOMAIN, you need to follow that up with some kind of check against a WHOIS server or root zone file (which is slower and harder to work with).
It's fine to use DNS queries as part of the checking process, but if you get a NXDOMAIN, you need to follow that up with some kind of check against a WHOIS server or root zone file (which is slower and harder to work with).