I am not necessarily a fan of tailwind, but the author is explicitly writing how to not use a design system.

Not knowing what m-3 is is the whole point. You use it across the website to have some consistency in your spacing, and not just slap around another 8px.

Yes, you can also do that with CSS variables, and Imho at times I do think that bootstrap for all it's faults is a better base design system, especially for internal admin interfaces.