Vlad I think having the feature would be better than not having it, even if it were not discoverable.
However, in the vertical tab pane, you could have an item "Open new child tab" on the right-click context menu for an open tab. I don't think this would confuse new users because they would already have to be using vertical tabs to find it.
There is also the fallback of making it discoverable through the Orion User Guide.
This feature would be a big improvement for me. I like to keep my tabs organized by concept, and having to drag tabs around all the time is kind of a pain. Better to open them in the right place at the beginning.