Until very recent price changes, at the scale that they purchase hard drives, I don't think that there would have been significant cost savings in purchasing SMR drives. Particularly after taking into account the additional hours of software development required to write custom host managed SMR stuff to deal with the performance problems.
Now that they appear to be using mostly enterprise SATA drives, I imagine there would be considerable cost savings, assuming they pull off a migration without serious problems. But that's a big assumption, and the risk certainly justifies a cautious approach so long as they remain quite profitable without SMR.
