Yea I do not recommend treating chromes prompt API as a good example of local LLMs. It's fine and stuff but it's really weak. 8b models from a year ago are better in some ways. And a lot of the recent model drops are meaningfully better.
Maybe it would do better with the new Gemma 4 models, which the Chrome devs have been hinting at moving to. And why the API doesn't let you introspect / pick the model, I'm still not sure.