I don't think they were saying they found out all NFTs must only be links, just they found out the NFTs being peddled usually were. As you say, bytes are bytes regardless what they represent - it's just a lot easier to grift something like a 32 byte hash or a link on a chain because storing all those bytes in the blockchain would have been a damned expensive way to run a grift.

There is a difference between markets for things I don't care about and markets for things I do care a lot about, just not in a positive way. I don't think it's necessary to allow an open market exchange for phone scammers, for example, and it has nothing to do with how uninterested I am in buying people's information. Not all NFT had to fall towards that kind of thing, it's just there was so little practical usage actually being done that 99% of it ended up being that way.