Boy, that is a real puzzle! I agree with your confusion. It's very weird. A total mixed-message. As Lizars1735 commented, maybe the socks will come out nicer with hand washing.
Superwash merino won't shrink, however, maybe the silk content will distort the socks. ???
Anyone else have ideas, or experience with this type of "superwash" yarn with a hand wash recommendation?
I'm gonna run over to Ravelry and type in the yarn type, and see what the knitters (who have projects using the yarn) have to say about it.
If I get any intel...I'll report back!