I think you have it right. The time from that race should not be used. Proof of time from an officially timed race is required. To my understanding (although not explicitly stated on the RD website), there are two components to that. First, the race must be officially (chip) timed. Second, the race distance must be certified (although there may be exceptions in some limited cases). In this case, the race was chip timed, but the distance was not certified. There have been varying estimates of the race distance, but nothing has ever been officially measured (nor will it be). I don't think this is an appropriate race to use for a proof of time under any circumstances.