Well, only it isn't.
Very old, unpatched and rooted devices can fairly easily pass device integrity check.
It primarily assures the software vendor that the phone is running Google buttplug in the privileged mode.
Remember, handsets running on ANCIENT versions of Android with no patches for years. Whilst seems to be important to raise under the Forbes article (rightly) fussing about a couple of zero-days.
"Custom roms" (whatever that means) can easily spoof the checks in the specific situation (mainly hardware that allows for several things).