It's not internal, but there are other private address ranges beyond 192.168.0.0/16. 10.0.0.0/8 is another common one. Container platforms like to use 172.16.0.0/12.
Oh wow.... Didn't know this lol. I thought that OP would be using their private IP to access their homserver from home. Turns out I got completely confused....
I think if they were using a private IP, there wouldn't really be a joke. Of course the router can resolve an IP in its network. The joke is that they're using their public IP from inside their network, and when the request gets the router, instead of resolving externally, it resolves to the public IP of the router itself.