I was planning my NFT project and had similar doubts. I thought that I would need to put all hashes into smart contract and that would cost a fortune, so was thinking if I could do that on some L2 etc

Anyway, I do understand why HM team did this, if they pushed each HM by themself it would cost a lot, they did raise a lot but maybe they didn't know if it would be like that and they would need to pay in hundred thousands which they didn't have.

My final thought is that they did bare minimum, but probably this is enough. I think it would be nice if you could take hash of your HM and check it against provenance hash if it is part of it, but I am not sure if this is possible? But the thing is that you can check any other provenance page against this hash added in SC and it wont fit, only the one that is on the web page is possible. This page is not in blockchain but neither are images, so basically it is combo of web3 and web2. They can also put provenance page to IPFS as they did with images. some parts must be stored off blockchain but all of this can be verified with BC. So basically storing each hash of image to chain would just provide convenience for checking hash against tokenId on chain but its not really something that proves more then just original hash.

Also found this article that could be interesting for reading more on this topic.

https://edition.async.art/blog/january-2021-guide-to-tokenized-art-permanence-and-provenance