All the other IDEs they have are variants of IDEA.

Fleet is a completely different codebase.

So they’re correct, there’s only two families of IDEs.

Isn't Fleet in preview 5 years later?

Fleet is cancelled, see the link above

Kind of figures, they were trying to compete with VS Code but they were not even able to create a stable release in all that time.

That’s a bit of an uncharitable take.

Fleet was very stable to use , it just never successfully turned into a product which they address in their link as well why that happened

It never left the preview stage, so they did not feel confident calling it stable enough for their users and I don't use preview versions for products where there are already 10 production grade competitors.

Uncharitable but yeah, reality isn't always charitable.