In general the purpose of upsampling is that you allow a simpler gentler slope analog filter to be used to get rid of any aliasing. For Redbook with no upsampling you need a filter that goes from full level at, say, 20k to next to 0 at 22.1k. That’s a very steep filter and it not really possible in analog without lots of compromises. If you upsample even just by 2, then you can go from full level at, say, 20k to 0 at 68.2kHz (88.2k - 20k). 4 times give you a final cutoff at 156.4k (4 * 44.1k - 20k)… There was nothing specific to the DS in my first comments. DSD allows a much gentler anti aliasing filter than PCM simply because it’s oversampled to a higher rate. There is a rise in ultrasonics if the filter has a high frequency cutoff or a shallow slope.
The high internal oversampling in the DS isn’t directly related to that - it just allows upsampling, say, 352.8k by a small integer factor. Going to quad rate DSD allows the ultrasonic energy to be spread over a wider frequency range and hence also lowering the amount of ultrasonic energy that’s not being filtered out.