Can you know the macOS version from the firmware version in Macs?
No, you can't.


Some Macs didn't get their updates. For a host of possible reasons. Install by image would be one where this is expected (Such as copying a partition from another Mac). Then the firmware is older than the expected OS.
Some Macs did get their updates with OS updates/upgrades. But then the owner decides that Mojave for example is still too buggy and they downgrade to what they had before. Then the firmware is newer than the associated OS.


What you get with the firmware info is just a weak indicator of what was installed at one point, but not a reliable one for what actually is installed.