I never used those so correct me if I'm wrong but those UML tools were more of a one way street: diagram -> code.

You couldn't point it at an existing codebase and get anything of value from it, or get it to review pull requests, or generate docs.

And even for what it was meant for, it didn't help you with the design and architecture. You still had to be the architect and tell the tool what to do vs the opposite with LLMs, where you tell it what you want but not how to shape it (in theory).