You can use AtlasKit / Design System components (yes, there is a difference) if you are willing to stick with React 16 and in some cases even Styled Components 3.2.6. We are using React 16 by default and haven’t had any issue. We’ve made it clear that any vulnerabilities that come out of having React 16 are the responsibility of Atlassian.
If you do need React 18 for some reason, there are Marketplace Partners that tweaked their build long enough to be able to ship both React 16 and React 18, allowing them to use AtlasKit / DC components in combination with 3rd party libraries that depend on React 18.
Now with regard to your question about open sourcing AtlasKit, you can read the response from Atlassian here: Atlassian, stop bullshitting and open source AtlasKit, UI Kit, Forge Runtime, AMPS, Atlas SDK, etc, etc - #2 by tpettersen