I would argue that the need to add "In" - to create contrast between the situation with reference to the past as opposed to that without such reference - only emphasises the fact that "retrospect" by itself does not imply hindsight. Its inclusion in a sentence indicates that things would be/seem different if the past were not being considered. Note that the phrase " In retrospect" does strongly imply hindsight. Any fresh evaluation would depend upon the intention and perceptiveness of the actor - it is not automatically implicit. Consideration is mentioned in places but reconsideration is not.
In contrast, the definitions of retrospect refer simply to the act of looking back. That is the essential nature of hindsight. The definitions of hindsight stress the new (changed/superior/matured) perspective gained after the event.
regard or reference to precedent or authority, or to previous conditions. Some dictionary definitions to clarify the difference: A sentence containing the phrase "in retrospect" often describes a new appraisal of the past time or event but I would not say it is essential. Retrospection can confer hindsight, but it is not part of the definition. Hindsight is a perspective on the past in which understanding has been gained which was unavailable to the observer at the time in question.
Retrospect is simply looking back at the past.