We're talking about space junk, right ?
"It's unclear whether a Detumbler could simply be slapped onto an existing satellite "
If we're talking about space junk, I would hope that this Detumbler is supposed to fix on existing junk. A satellite that is still functional has attitude control, and thus doesn't need a Detumbler.
That said, Airbus hasn't said how it is supposed to get the Detumbler on the cubesat for the test, but I hope that said Detumbler will launch after the cubesat, to catch up with it somehow and latch on magnetically. If it can indeed orient itself, lock on target and basically dock, then it would be a massive success and we could start thinking about sending scores of Detumblers to help bring the junk down.
Of course, that sounds too good to be true, so . . .