Much of the complexity is likely in building and managing such a large botnet.

There also isn't a lot of details here on the exact nature of the traffic. They say it was hard to distinguish between legitimate traffic and this malicious traffic. So the botnet is at least rotating their requests through lists of customers hosted with them (though that isn't complex, but it is forward thinking. If the botnet was all making non-stop requests for just a few domains, that would be a strong signal to start filtering traffic, first internally, then pushing ISPs to block it upstream).