I can’t say with 100% certainty but I recall reading a post from ST staff to that effect. My own hunch is that the hub treats any network outside of its’ own subnet as external. If it didn’t the hub could theoretically be used as a tool to attack other networks through a simple SmartApp.
I was going to say, if you know what you’re doing and have a device/router capable of doing it, just NAT it from the ST segment to your destination segment and then reference the NAT instead, that should work.