Yes, DNSCrypt is in fact a point-to-point tunnel, so it's like encapsulating DNS queries in Elliptic Curve Cryptography provided by curve25519xsalsa20poly1305. See DNSCrypt Proxy TECHNOTES.
Regular DNS servers are not equipped to speak DNSCrypt but OpenDNS ones are. So when you change the DNS IP and use DNSCrypt then your dnscrypt-proxy would first try to exchange a DNSCrypt certificate before it accepts talking to that server. A regular DNS won't reply and you won't get a question/reply going. This explains why it doesn't work for other servers than OpenDNS.
However, there are different DNSCrypt providers. You might want to look for them because they vary. Some do not store logs, most of them are free of the shenanigans that OpenDNS does with non-existing domains, some offer malware filtering capability, etc.
List of alternative DNSCrypt servers: https://github.com/jedisct1/dnscrypt-proxy/blob/master/dnscrypt-resolvers.csv