I have the same issue. I have an old Vue 2 code that I am migrating to Vue 3 and this is a stopper for moving further.
The fix seems simple and should work. When someone can review it and make a patch?

This shouldn't block you since you can always just cast Icon to DivIcon.

I am returning the icon from a function my code looks like this

const getIcon = (): Icon => {
  return L.divIcon({
    IconParams...
  }) as Icon
}