Ideally, if “tabs are enabled” (and, possibly, the Always show tab bar option is ticked – if you feel this provides better backwards compatibility for the user experience), the URL?reveal=1 link should open the referred document in a new tab. Alternatively, you may want to consider a reveal=2 option for specifying that the referred document should be opened in a new tab in the main/existing window.
Whilst opening in a new tab can be achieved from within DT using Cmd+Click (or via the context menu), there is no mechanism to engage this behaviour from outside DT (e.g. if using x-devonthink-item:// links in third-party documents).
As a final comment, the current behaviour of activating a tab that is already showing the referred document is excellent and should (obviously) be retained. However, there is currently a defect associated with this implementation: the currently active tab is cleared. Other tabs are preserved correctly; only the active tab is cleared when activating the tab that contains the document corresponding to the x-devonthink-item:// link.