how can an isochron map of the ocean seafloor support the theory of seafloor spreading?

One notices that the various magnetic stripes are lined up parallel to segments of the nearest mid-ocean ridge, and that there is a symmetry in the patterns of stripes on the two sides of the ridge. Like, if the 5th stripe back is the widest one on the east side of the ridge (I made that up), the same will be true on the west side of the ridge. This is consistent with the idea that the ridge itself is the source of new material that is pushing the two sides of the ocean floor farther apart with time.

The notion of an "isochron" map makes no sense until we ALREADY believe in the theory of seafloor spreading! That's why my paragraph above mentions only the alternating stripes of opposite magnetic polarizations.