i dont understand the author's point. is he trying to say Svelte components dont allow children? because thats what slots are for.

Swyx: excellent insight. I talked to Rich Harris about this article. Most of the issues are no longer true. The sole exception is that, yes, React can copy a DOM subhierarchy quickly, and Svelte cannot.
However, this is the functional (as in Reactive) mindset.
You pointed out the OOP approach, which is to parameterize a reusable component with slots!
Perhaps there is still a case, where reactive late binding does not work as well in Svelte's eager optimization, but I cannot yet think of a good case.