The HDX should not require a TIP license. Since 1.7.4, CTS endpoints support standards based SIP interoperability ("Native interoperability" in our docs and release notes). Check out the CTS endpoint interopability page here which has some tested configurations for calls between CTS and HDX:
It shouldn't matter where the HDX is registered, since CTSs can now make P2P calls with any standards based (SIP / H.264) or TIP endpoint. I would guess it's registered to a VCS in the demo; that seems to be what we see more often.