Yes, we've never really cracked doing vi's (Or Vim's) command mode in the difference editor. For one thing, you get into command mode by pressing Esc, which closes the dialog by default, but that's just the start. For each command you run, it gets mapped to a special diff version. This is for a number of reasons, not the least of which is trying to keep undo in sync between the buffers (which still isn't perfect).
But it would mean mapping all the command mode vi keys to diff versions, and some may not have equivalents (for example using "2D" to delete two lines). Plus, there really hasn't been a lot of demand for this over the last 21 years that we've shipped DIFFzilla (although it was just "diff" for a while
) And this doesn't even mention the ex command line, which SlickEdit does a very good job emulating and includes another set of things that would have to be re-written for diff.
I'd be happy to try to figure out a tweak for you so you could use '/' to search, but A) it would probably have to launch the gui_find dialog, and B) I'm not sure how you would insert a '/' if you needed to (you can keep typing in our diff output).
I know it's not the same, but it looks like ctrl+alt+/ is not bound in Vim mode. You could bind that to gui_find.