Yes, your understanding is correct - see the post by Daniel below. I asked this back in May, between then and now things may have changed so it may turn out a bit different. But this is certainly very exciting as far as I am concerned.
"However, the intention is certainly that you will be able to use third party VST instruments and effects with Dorico. You will have direct control over which device, channel, patch (and other things like controller, keyswitch, etc.) should be used to play each playing technique played by each voice of each instrument, and that might mean that they are all played by the same device, all of them played by different devices, or anything in between.
The idea behind the MIDI editor in Play mode is to allow basic graphical MIDI editing; the intention is not to replace Cubase, but instead to give users who are savvy about how to bring virtual instrument playback to life the tools to do at least some of that work in Dorico, rather than forcing them to move to the sequencer or DAW right away."