Well while this is not indisputable proof there is a very relevant discussion on virological.org related to both Furin cleavage sites and orgins of SARS-COV-2. The damning part of the leaked research proposal is that they ALREADY had partner researchers located near caves in three different parts of Yunnan Province.

For a more detailed discussion of the origins of SARS-COV-2 while not directly addressing the leaked DARPA proposal please see: https://virological.org/t/tackling-rumors-of-a-suspicious-or...

They also discuss that the base virus was mostly likely linked to bats in Yunnan province. How it got from Yunnan to Wuhan is a matter of question.

However, if the research was being conducted in the US and accidentally leaked into the population it may have been in circulation for sometime (months) before causing serious illness or disease (mutations).