Good question. Why would "bit perfect" software be able to change sound quality in the first place ?
surely if bits (or words) are clocked out of a buffer exactly as they were clocked in then the time spent in the buffer should not affect sound quality.
Whether that would be true or not is less important, because : No such thing (from your quote) exists;
The clocking out (or reading out) is not under the direct control of the PC; the DAC causes that. When the buffer runs empty that can be seen, and that indication fills the buffer again (this *is* under control of the PC hence the playback software).
Peter