But why not just use an email address or something for that case? I get how it would be ideal (and myself desire) to have one messaging tool which works in all cases, but it doesn't seem like a massive failure specifically of Signal to not work for this usage. Certainly not one large enough to warrant the torrent of comments I see any time Signal gets mentioned on HN

Sure that makes sense to me, however I will say that if I was a vendor selling stuff internationally I probably would want (and even require, depending on export controls) some level of de-anonymization on the buyer's part. Presumably at the very least you need to provide a shipping address to receive the goods, which is no less identifying or difficult to spoof than a phone number.