I suspect that because Sony is a Japanese company, and has their headquarters in Japan, likely has most of their important datacenters in Japan, which unlike the USA, has incredible internet speed, and because Sony is a tech monster, likely has some pretty serious connections to their stuff.
Now couple that with what we have already seen of general network incompetence with the last huge Sony breach to their Playstation network, due to them simply not updating their software to a version several years out of date, I don't think it is all that surprising.
However you are right, 100TB is nothing to sneeze at, and would take some time, and likely multiple connections to work. I suspect that Sony was clueless about what was going on, until someone complained about slow network connectivity, and eventually some sysadmin started looking at things, and started to see connections, and bandwidth saturation, and then trying to figure out who was doing it, and on finding it wasn't Sony, needed approval about severing the connections (if even technically that easy)... and once approvals and technical fix were done, well 100TB is gone.
I suspect with the amount of interconnectedness of distributed networks, it wasn't as simple as walking outside with an axe.