That’s a fair assumption based on how Alexa works for me and the OP’s original post(s), but…
My child devices are working in Alexa, and those device handlers are ST stock. Even though the parent device is Zigbee using a custom DTH, the child handlers are agnostic, such as the DTH called “Child Switch” (and others).
I don’t do anything with motion, but the OP’s other post discussed basic voice control for his lights, so I didn’t elaborate, or how using a virtual device could be used instead.
With regards to limiting what devices are exposed, that’s technically not the point of the OP’s original question/concern, which was just getting Alexa and ST to work together. I personal hate that I can’t limit what’s exposed, but I am hopeful that ST will come around and give us that control back again. Same thing applies with being able to select which location.