I don't know why, but I thought this was going to sandbox style tab/split support for the all the baselines macos apps.

This is very cool, but somehow got myself disappointed that something I didn't know I wanted doesn't exist.

You're not the only one. I first assumed it was a library when I was scanning the headlines, but then when I started opening up tabs moments later I thought it added tabs and splits to existing apps. I remember something that brought tabs system-wide to Windows so it's not even too crazy of an idea.