You're spot on about the PLL stage, a few people on Reddit pointed the same thing out when I first posted it. Makes sense, although I still think it's interesting they added it internally to an otherwise (apparently) cloned design.
You're also right that USB CDC does provide the option for a generic USB Ethernet device, however this silicon is ASIX-specific (not just the USB IDs.) ASIX's Windows drivers include their own system driver binaries, and the ASIX Linux driver has a lot of ASIX-specific stuff in it.
I think it's kind of possible ASIX made this themselves as some kind of no-name branded unadvertised market segmentation effort. I can't understand what their rationale would be exactly but hardware companies do unusual things sometimes...