The docs for the Device Tampering Verification node (https://backstage.forgerock.com/docs/am/7/authentication-guide/auth-node-configuration-hints.html#auth-node-device-tampering-verification) state:
The Device Tampering Verification authentication node inspects the captured metadata of an authenticating device and assigns a numeric score based on whether it has been tampered with; for example, if it has been rooted or jailbroken.
A score between zero and one is given to the device, based on the likelihood that is has been tampered with or may pose a security risk. For example, an emulator scores the maximum of 1.
My understanding is that it is the device that calculates the score and sends it as the value of the attribute jailBreakScore. Then the node simply uses the value to compare with the threshold.
Also, the Android emulator returns 1 indeed, but the SDK docs state that iOS emulators return 0 (I did not test on iOS).