question on wheeler feynman absorber theory

In their original paper the authors derive a phase between the
acceleration of the source and the advanced wave at the source coming
from the absorber. This phase is needed in order to conclude that the
reaction is proportional to the time derivative of the acceleration
rather than the acceleration itself.

To arrive at this phase however the authors argue that while the speed
of the retarded wave is reduced by interacting with intervening medium
between the source and the absorber particle in question, the speed of
the advanced wave on its way from the absorber particle to the source
is the speed of light and does not interact with intervening
particles. The reason for this they say is that unlike the retarded
wave which they treat as the combination of the primary retarded wave
and the secondary waves that come about from the interactions with
intervening particles, the advanced wave is an ELEMENTARY interaction
between the absorber particle in question and the source and therefore
does not interact with other particles. What is this difference
between the treatment of the advanced and retarded wave motivated by?
Why is one considered elementary and the other not? Isn't this a time
asymmetric assumption? If the answer to the last question is yes then
why do we need to blame the time asymmetric dominance of diverging
waves over converging waves on asymmetric initial conditions why not
also blame it on this asymmetric treatment of advanced and retarded
waves?

Help would be greatly appreaciated!
Thanks

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