Well, they won't. All NFTs are hosted on hilariously centralized services like googleusercontent.com and similar.
Browse through https://opensea.io/, the most prominent NFT marketplace at the moment. Digging through the DOM of their website will reveal a lot of links to quite standard content storage solutions. They usually go through a bit of trouble to prevent people from simply clicking "view image in new tab" to reveal the actual location, but it's not difficult to get around.
Almost all NFTs are hosted on IPFS. Opensea shows a CACHED version of the image, because pulling them directly from IPFS puts unnecessary strain on the network and is comparatively slow.
The contract for the NFT generally just points to an IPFS identifier (a sha256 hash of the content), and it's up to the viewer (a website like opensea) to decide what ipfs gateway to use. Even if the ipfs network died you could still use other p2p methods to find the content, as long as someone was still hosting it.
The amount of artwork NFTs that has "Metadata: Centralized" in their details appear very high, although it's difficult to get any exact numbers. As far as I can tell, OpenSea doesn't provide an option to filter based on this field.
Some high value collections like BAYC have frozen metadata, so it can never be changed. I do know some other big collections serve images from AWS or their own domains, this happens because it's extremely expensive at current gas prices to do partial reveals or any kind of metadata changes on-chain
It's a problem, it used to be the grand majority of collections were fully decentralised and frozen but nowadays it's a mixed bag – I think it will change once the gas problem is solved, but it's also partly because the audience has changed and newcomers into NFTs don't care so much about their tokens being fully decentralised.
Did you actually read the post? They propose to put it on-chain, and even say
> [O]ur NFT collection could very possibly become the largest on-chain collection ever in the history of Ethereum. Time would only be on our side because as gas becomes more expensive, the window of opportunity to bypass our on-chain NFT collection would fade away.
Remember that this is from the people that believe buying a book gives them the IP rights to the work (should have bought a Harry Potter book). They may not have thought it all the way through.
To give an idea of the storage costs associated, I dug up this old stackexchange-answer about storage costs on Ethereum [0], which estimates that it costs about 76.000 USD / GB stored. Note that this is 5 year old answer, so the price will probably have multiplied like the price of ETH since then.
Edit: Here's [1] a newer, updated price estimate that takes current prices into account : 309.9 Million / GB. LoL!
Browse through https://opensea.io/, the most prominent NFT marketplace at the moment. Digging through the DOM of their website will reveal a lot of links to quite standard content storage solutions. They usually go through a bit of trouble to prevent people from simply clicking "view image in new tab" to reveal the actual location, but it's not difficult to get around.