Could it be made less bad if you could turn the tracker off (switch to airplane mode by physically removing power from the modem) any time you want?

It'd be nice if the PinePhone supported this.

Both the PinePhone and Librem 5 have physical kill switches for the modems.

I do not follow the car analogy, please explain.

Not GP: but I assume the answer is "because GPS is in the modem, the closed source untrusted part could still be actively disclosing your location".

.. But the modem itself is broadcasting it's identity to multiple radio receivers. Is there enough of a difference wrt GPS positioning for the distinction to matter?

Sure, I agree it’s subtle. But it is: a) much higher resolution b) potentially to a specific third party who may not have access to cell network data. Like, say, an angry ex-spouse.

Aha! That is a valid concern indeed. Lesser than the usual "baseband has full access to whole system", but still significant.