Well, we can make some pretty accurate guesses about the frame of reference if we know the nodes' geographic positions and ban airplanes.

yes but those guesses have "smudge windows" which are functions of the physical distance between relevant nodes

if you want to make assertions about order between events from a set of nodes, and you want to use per-node physical clocks to determine that order, then even if those clocks are perfectly synchronized, order can only be decided when the light-cones of all nodes intersect, which is the maximum distance between any two nodes times the speed of light

7ms works for up to 2098km, that's in the best case