Correct, sort of. no subpoena required to utilize Mobile Ad ID data just as Fog Reveal does for your local, state, and federal law enforcement departments.
I think/assume that your wireless carrier (VZW, ATT, TMo, etc) DOES require a subpoena as this is essentially "wiretapping". GrapheneOS specifically enables us to block MAID tracking and App specific tracking.
From Fog article:
"What distinguishes Fog Reveal from other cellphone location technologies used by police is that it follows the devices through their advertising IDs, unique numbers assigned to each device. These numbers do not contain the name of the phone’s user, but can be traced to homes and workplaces to help police establish pattern-of-life analyses."
I would assume the Google Fi utilizes everything they can, perhaps even cell tower data, to track your every move (but this is assumption.