Yes, it’s still true.
https://support.smartthings.com/hc/en-us/articles/204392790-Z-Wave-General-Info
When the SmartThings Hub is added into another Z-Wave network, it may not receive notifications from some battery-operated devices. SmartThings Technical Support does not provide assistance to consumers using the Hub with other Z-Wave controllers.
The fact that they won’t provide any tech support if you try it indicates they don’t think it’s going to work very well. But they can’t tell you not to do it because they are a certified zwave controller. So it’s technically possible, but results may not meet expectations. 
Have you had a chance to read the community FAQ?