Here’s the faq:
FAQ: Can I trigger an Echo Action without Speaking to It?
You create a virtual device in smartthings and then, when you turn that on, it can run a Alexa routine (not a smartthings routine) to turn a device connected to Alexa, but not visible in smartthings, on or off. Like a Cync bulb. Or do anything else that an Alexa routine can do.
Here are the Cync instructions
Cync Product Support Help Center.
That said, at the time of this writing, it’s a one-way integration. If you turn on the Cync device in any other way, including from its own app, SmartThings won’t have any way of knowing about it, so the status in the smartthings app can get out of synch.
So people more commonly use this method for either smart plugs or devices like robot vacuums, where they just need to turn it on or off. Bulbs get trickier, because there are so many ways to start them and you usually want to track the state.
Cync intends to put out new models that will work with matter, in which case, those will be able to work directly with smartthings as well, but they are not, as of the time of this writing, going to update the older devices. 