If you don't use namecheap's dns servers, dns requests for your domain do not go to namecheap for redirection. They don't go to namecheap at all. Your specified

If you don't use namecheap's dns servers, dns requests for your domain do not go to namecheap for redirection. They don't go to namecheap at all. Your specified dns servers get registered with the top level domain servers.