They claim 4K/120 on the box... it was obvious it had a Compatible cable...
The confusion was caused by the cable bearing the branding of a slower HDMI speed that's not certified for the bandwidth required for 2.1 HDMI features (8K/60 + HDR and 4K/120 + HDR). For reference, Xbox Series X ships with a cable with the right branding for 2.1 which made this even stranger.
It's pretty obvious that this absolutely would cause conversion and prompt questions, Sony have said that no, it's actually a 2.1 cable but no-one has had the common sense to ask them why the cable is marked as a slower speed. I'm sure there's good reason (see posts above for a good example of what might have happened).
To demonstrate why this caused confusion, here's a made-up example with different cable that we're more familiar with but the exact same steps:
Sony says new console is USB 3.
Sony includes a cable clearly marked as USB 2
People ask why a USB 2 cable has been included not a USB 3 one.
Sony says "actually, it's a USB 3 cable"
Some websites start to claim that 'USB 3' isn't common branding for USB 3 cables and actually USB 3 cables are marked as USB 2.
USB 3 cables do actually carry markings that indicate they are USB 3, website is incorrect but apparently it's the only rationale they could think of as to why Sony would say a cable marked as USB 2 is actually USB 3.
No-one thinks to actually ask Sony why the cable is incorrectly marked.
Era decides asking questions about why cables are incorrectly marked is concern trolling.