It took mere moments for 911/Emergency to override location preferences set on GOS phone. Phone location can be triangulated regardless of user location preferences, but my guess is that 911/Emergency gets very special privileges. What can else can 911/Emergency override on GOS phone? I think having a separate list of permissions for mobile carriers and 911/Emergency should be posted. There is some technical information on Android site, but a non-technical down-to-earth explanation is a great idea for GOS team to consider.
So 911/Emergency can override location settings... What else can it do?
Like you write, you were probably just triangulated. Depending on the amount of cell towers in your area, this can be fairly accurate.
GrapheneOS does not support AML (Advanced Mobile Location).
https://discuss.grapheneos.org/d/87-advanced-mobile-location-aml-support
At least not yet: https://github.com/GrapheneOS/os-issue-tracker/issues/1174
I don't know if GrapheneOS support ELS (Emergency Location Service).
At some point I saw a different thread about AML, but I can't find it. There GrapheneOS explained a little bit more about it.
An actual notification on my phone appeared during 911 call and said something about "Location Override". I just freaked out and erased it, but all possible location settings/toggles were disabled before the call. After the call, some of them, like the one for Phone app, became re-enabled.