My initial intuitive answer is a combination of Yes and No, however I also looked up Gestalt Therapy: Excitement and Growth in the Human Personality and here's how they respond...
"In dealing with intojects we again make use of the same techniques of concentration and development which were applied to retroflections but there is a crucial difference in procedure. In retroflections both the retroflected act and the behavior which it holds in check are parts of one's own personality, and what must be done is, first, to accept and identify with both parts, and second, to find a new integration in which both parts obtain overt expression. An introject, on the other hand, consists of material - a way of acting, feeling and evaluating - which you have taken into your system of behavior, but which you have not assimilated in such fashion as to make it a genuine part of your organism." (PHG page 189)
So the difference here is ... I feel like hurting you but also not, so I hurt myself as an option (retroflection)... and I feel like hurting you but tell myself I should not and so I don't (introjection).. and perhaps retroflect in the process.
I guess the difference for PHG is that they see retroflection as an authentic or genuine attempt to deal with two conflicting aspects of our nature while introjection involves operating on what has been taken in without a chewing over or assimilation (top dog?). Hence retroflection happens as a response to genuine responses which are in conflict and introjection is a "shouldism".
Now that's one hypothesis and the other possibilty is that retroflection can be to turn contact in on oneself no matter whether the cause is a conflict between authentic polarities OR because of an authentic need versus an introjective "should".
In this second possibility it does not matter whether we retroflect in response to a genuine conflict or in reponse to a top dog "should" (eg don't be angry)... either way we turn the contact back on ourselves.
Personally I like this second option better.. to determine retroflection from the direction of disruption of contact (ie back to oneself) rather than look to the cause of the turning back (because of a genuine conflict or a top dog introject).
It is interesting to note that PHG also say...
" For projection to occur, retroflections and cofluences are also necessary, just as they are in introjection; and, in general, as we have said before, the neurotic mechanisms are all functionally related to one another and interlocking." (PHG page 211)
So perhaps BOTH/AND after all :-)