Maybe read through the documentation and try it out. It has very robust configuration options, many pre-built modules that are ready to include in your prompt, for example, one that prints git info for the current location if it's a repository.
So yes, it is kind of a competitor to p10k, but not zsh. It's just the prompt, and it just focuses on being a very good prompt tool.
Have you tried writing a PS1 which shows all the info that's demonstrated on the linked page?
I have. It's a decent chunk of work. It's a whole lot more work to do it well. Did you remember to handle the situation where the git repo is so gigantic that running `git status` takes half a minute? Or the case where there is a .git folder somewhere but permissions are wrong or it crosses a mount barrier so `git status` prints a page of text to stderr? Does the way you're finding the current branch handle the case where HEAD doesn't point to a branch?
The only way your prompt "already does" this stuff is if you've spent a whole lot of time configuring it and tuning it, or if you're already using something like oh-my-zsh. And if you're already using something like oh-my-zsh, well, starship is just a competitor to (the prompt portion of) that.
People want something easy to use with good defaults. If that wasn't the case, shells wouldn't have a hundreds of plugins dedicated to customizing the prompt.
So yes, it is kind of a competitor to p10k, but not zsh. It's just the prompt, and it just focuses on being a very good prompt tool.