I haven't yet read up enough on NFTs to confidently answer your question, but I'm pretty sure that they work a bit different from what you described.
Also, I already came across 'Bitcoin NFTs', which are sold on https://raretoshi.com/. While it's 'on BTC', it actually uses the Liquid sidechain.

But it is definitely possible to implement NFT directly on Bitcoin; simply through using a coloured coin.
To be honest, we have these for years and they can directly act as an NFT without any modification to the spec.