What does this equation mean?

This is not the same kind of zero-over-infinity problem that shows up when comparing two accelerated trajectories, and the difference matters. For an eternal accelerated trajectory, both the numerator and the denominator of the naive, un-normalized response functional are proportional to the (formally infinite) total elapsed proper time, and that shared infinity cancels cleanly in the ratio, which is exactly the standard trick — divide the total accumulated response by the total proper time elapsed — used throughout this literature to extract a finite rate from an eternally switched-on detector. For the eternal inertial trajectory, the numerator does not merely fail to diverge fast enough to survive the ratio: it vanishes on the nose, for every gap, at every order. There is no shared infinity to cancel against, so there is nothing left for the ratio trick to rescue. $D_{\rm det}$ against an inertial comparison worldline in this idealized limit is not small. It is not defined.