I don't believe that dd writes to a disk buffer in RAM.  Certainly, the drive itself may use a write buffer, but writing the entire disk would easily exhaust that buffer.  I don't know why they would want thousands of passes though. They may have had a point back in the days of MFM hard drives, but encoding schemes have become very sophisticated.  Modern hard drives must employ extensive error correction codes just to be able to read the last written contents of a sector reliably, I stand by my contention that alternating 0s and random junk for a total of a dozen passes should render the data completely inaccessible to anyone (with the possible exception of those spy agencies, and I'm not even sure about them).