Dropping a file on the icon in the dock does not trigger the On Import trigger; I’m not aware of any trigger which triggers when you import this way (@cgrunenberg is that intentional?)
The On Import trigger will fire if you drop the file via the sorter or directly into the Inbox in Finder (assuming you have activated the add-on Global Inbox in Save Dialogs), or directly into the inbox in the DT window.
I hadn’t actually noticed the behaviour to date - but I recreated your rule to see what was going on and played a little bit. I actually did try the other possible triggers just in case, but dropping on the icon doesn’t ever seem to trigger the smart rule. We’ll see what Criss says - it could be a bug.
I tried it too and guess what—the trigger On Import worked for me when I dropped a file on DEVONthink’s Dock icon!
Or at least it did so as long as my action was Display Alert (an easy to detect action I always use when I’m trying to find out if a Smart Rule was triggered properly). Adding a tag worked too.
But On Import did not work with Change Name. So the interaction of these two does not work and not the On Import trigger in general.
Is the group selector enabled in Preferences > Import? If it’s enabled and only one file imported, then the title entered in the group selector is used. Therefore either disable this option, drop multiple files or remove the default title in the group selector in case of one file.