This is a very good point. The truth is that we just don't know how many Sumatran rhino are left, they seem to pop up every so often in the news in different spots around Borneo and Sumatra. As stated, I am optimistic of scattered mainland populations of one or both of them.
Of coarse there are risks having all the population in one place but at the moment this is working well as the focus against poaching, As in the case of the Sumatran rhino a scattered population is much harder to protect also when the last mainland Javan rhino were discovered in Vietnam in the mid 80s it did not take long foe the poachers to move in and mop up the last dozen left living wild. Hopefully a second home can be found for any excess animals to be moved out from the current location but its going to need the same protection level as what they currently have.
There is now an estimated number of 77 Javan rhinoceroses, according to this link. It is exciting to see little increases, as it sparks hope for the future of the species!