I take exception to people using the term "design system" for what is essentially a style guide and a bunch of components.

As I wrote in my Tumblr blog (https://tmblr.co/Z-VHVyY4sGrVay00): "A design system is an organizational tool, bringing together design principles (when do we use cards? when do we use buttons vs links?), components, layouts, and agreed on decisions on accessible design and markup. Systems are built and maintained by designers, developers, and product managers. Yes, color palettes, typography, buttons, and form elements are crucial building blocks. They help avoid having 15 different grays, or different padding and spacing from screen to screen. But they are not a system. Systems involve people and processes."

Design systems are specific to a particular company and its product team; they are part of a company's voice, design philosophy, and engineering infrastructure. Hence the word "system". This has far far less to do with being "pixel perfect" than with building a system of components based on a set of design principles, and that integrate with the company's engineering platform and templates.

UI kits are just that, UI kits, no matter what they're called. Do they perhaps provide a jump start to building a design system? No doubt. But they aren't systems until they're actually part of a system.