ERC-721 – Why Crypto Art NFT Platforms Set Low Max Supply for Minting Tokens

I can't find that particular piece in their contracts, but the below applies to the 2 of 10 edition.

How can I view the URL link to the associated image file as it appears within the metadata/code of the NFT itself?

The 2 of 10 edition is here.

From that we see their contract - MakersTokenV2 - is at 0x2a46f2ffd99e19a89476e2f62270e0a35bbf0756.

You can use Etherscan to read data from the contract here.

The token ID is 42497, which we can use with the tokenURI() method from that Etherscan page, to get the following as the location of the artwork's metadata (not the artwork itself):

ipfs://ipfs/QmdNvqCbd7qYvtyFJdd3zoJC84cvyCiSxYDfsiYYQZVCec

Which is an IPFS path. To view it through a browser requires you to use a gateway:

https://ipfs.io/ipfs/QmdNvqCbd7qYvtyFJdd3zoJC84cvyCiSxYDfsiYYQZVCec

From there we can see the image's metadata, which includes the actual location of the art:

{
  title: 'Trajectory',
  name: 'Trajectory',
  type: 'object',
  imageUrl:
    'https://ipfsgateway.makersplace.com/ipfs/QmNopeEan1r728EGpAYZw8fFvCXfuXtnadC3JCeY5mo9LB',
  description: 'Created by artist ThinkLumi',
  attributes: [
    {
      trait_type: 'Creator',
      value: 'Think Lumi',
    },
  ],
  properties: {
    name: {
      type: 'string',
      description: 'Trajectory',
    },
    description: {
      type: 'string',
      description: 'Created by artist ThinkLumi',
    },
    preview_media_file: {
      type: 'string',
      description:
        'https://ipfsgateway.makersplace.com/ipfs/QmNopeEan1r728EGpAYZw8fFvCXfuXtnadC3JCeY5mo9LB',
    },
    preview_media_file_type: {
      type: 'string',
      description: 'jpg',
    },
    created_at: {
      type: 'datetime',
      description: '2020-05-06T18:24:35.578285+00:00',
    },
    total_supply: {
      type: 'int',
      description: 10,
    },
    digital_media_signature_type: {
      type: 'string',
      description: 'SHA-256',
    },
    digital_media_signature: {
      type: 'string',
      description:
        '72fd9dc05cc5b630c28d749eb56dc4f99173e6769556f7ac10c8204dd1de8721',
    },
  },
}

As Ismael points to in the comments, it would seem that the author of the linked comment in the OP's question was not pointing to SuperRare being on-chain and Foundation off-chain, but was emphasizing that the value of SuperRare as a curator. The "on-chain" provenance comment likely means that being able to point to an NFTs on-chain history means it can be traced back to where it was purchased from on-chain, and having been in SuperRare is part of what one is purchasing when they purchase such an NFT (in the thread author's opinion.)

There's something we should really unpack here as an additional note on the question: NFTs, according to the ERC-721 (and 1155) standard that they are based on, do not (generally) "include" a JPEG or other image. There is generally a field in the NFT that contains some kind of pointer (if you look at the standard, this is the extension called the Metadata extension) traditionally a URL or IPFS (or similar) hash, to a JSON. The JSON, in turn, links to the media. This is very significant, as it means in many cases that the media can become either unlinked (in the case of the JSON pointing to a centralized IPFS gateway, for example) or even actively changed (in the case of it pointing to someone's traditional web site, for example). That's a bit tangential, but very useful in terms of understanding NFTs.