Looks from the description as though it should work, you probably have a duff one. But what was wrong with plugging the guitar directly into the Mac audio line input? The cable would be a lot cheaper.
The merchant readily agreed to send me a replacement, without even wanting me to return the first one. I can't help thinking that he probably knew that these cables are dodgy. So, let's see if the new cable works, when it arrives.
I don't know why my son did not use the audio line input. He might not necessarily have known that this was possible, or perhaps he did not have a suitable cable. Based on a very quick Google, I see that using the audio line input is apparently not great quality, so perhaps he hoped USB would be better.