I’m not sure anyone has mentioned this yet, but there exists a painful workaround through a trusted third party like Alexa, IFTTT, etc.
One of the two (either HA or Smarthings) needs a virtual device, and the third party needs a series of automations to keep the virtual device in sync with the real one.
a. If the virtual switch turns on, turn on the real light.
b. If the virtual switch turns off, turn off the real light.
c. If the real light turns on, then turn on the virtual switch.
d. If the real light turns off, then turn off the virtual switch.
It’s kludgey at best, introduces noticeable latency, is a nightmare beyond a few devices, and I haven’t fully tested it yet. But…in a pinch with one or two devices…I’ve previously used it to keep two otherwise-incompatible services in sync with each other.