Sorry, I'm not sure I understand your comment. You can presume whatever you want, but I'm telling you how it works. :)

IIRC there are plans to switch Chromium to its own certificate store on all platforms, but they seem to be a ways off.

When you add a root certificate to a browser, typically it is configured to accept BOTH the added cert and the built-in/system certs. There would certainly be no reason not to in this case.