Tailwind UI looks like a proprietary product by the creators of Tailwind CSS.
It's more like a series of templates. You copy them into your project and customize them. I used them for a big project and found them to be absolutely fantastic. It's a lifetime license for unlimited projects - the time I saved was very quickly worth the cost.
Define proprietary. You pay, they provide full HTML (with Tailwind styles) for all of their controls, that's the point. Worth the money IMO.