It shows five carpals, one of which is evidently a degenerate semilunate. Didactyly is pretty easy to develop in parallel. Ornithomimosaurs also show a degenerate semilunate.
Tyrannosaurs also show a good many more maniraptoran autapomorphies, particularly in the skull.
It was as long as metacarpal II and quite slender. Well, ornithomimosaurs show many attributes which put them closer to birds than dromaeosaurs, which would limit membership in the Maniraptora to the birds alone. Arctometatarsalians may well, in fact, belong to about the same evolutionary grade as the enantiornithians, reducing the membership of the Maniraptora down to the Ornithurae alone. If you can't convince them, confuse them.