That's true. For a number of reasons, it is not possible to crawl a network like Gnutella completely via TCP if your resources are limited, that's why LimeWire implemented an UDP crawler message a while ago. The ability to use UDP pings for crawling hosts has finally been added to the official crawler.

I still don't quite see, how LimeWire is calculating the total number of hosts, though, because:
unique leaves + ultrapeer count > total host count.