Capn Bloggs : I think you have it most probably .
The high rate is always a problem. When receiving a level off RA , you disconnect A/P and if you do this close to the selected altitude your chances or overshooting a few 100 feet is very high .
This is one of the cases where the incident is in fact caused by having TCAS . No TCAS and the A300 would have most probably leveled off at FL210 as cleared by ATC and (hopefully) selected in the FD by the crew . The RA changed that a bit . But TCAS assume the intruder will not level off , In fact it is there to cover you for the " most probably" and the "hopefully" which is a good thing.
We just have to live with those once in a while.