Correct me if my understanding of internet routing is wrong, but the IP blocks were not out of service during the experiment. Only that a router(s) was advertising that it would accept traffic for those addresses. It would cause traffic to be sent through AS15562 during the time, which may have been suboptimal at times, but eventually would have reached the destination without visitors being aware of the game being played behind the scenes.
Your understanding is mostly correct. Whether or not traffic directed to any of the /24s involved "would have eventually reached the destination" is undetermined (and possibly irrelevant), meaning it's both possible and not possible. We simply don't know. Speaking strictly about "internet routing" (BGP in this case), it is 100% possible for an announcement to send traffic through an AS which literally dumps it on the floor -- it's happened many times over the years (the Pakistan/Youtube one was noticed by many): https://en.wikipedia.org/wiki/BGP_hijacking#Public_incidents
The question is whether or not the /16 (of which most of the advertised/withdrawn /24s make up) was used by actual devices, or if it's address space NTT has yet to use. If it is assigned to NTT but unused space, then effectively no harm done. If it's actually used IP space, then that would be very inappropriate.
