I think this is a fair presentation of Łukasiewicz's view on past, present, and future statements in an answer by Johannes https://philosophy.stackexchange.com/a/31995/29944: "His view is that statements about the past and present have an unalterable truth value, so if they are true they are necessarily true, if they are false they are necessarily false. Future contingents are assigned the value i, those statement are possible."
But if I were to claim Alexander the Great fell off his horse on his 12th birthday, is that statement "necessarily truth" or "necessarily false"? We have no way to know whether Alexander fell off his horse on his 12th birthday. Hence, it's neither true nor false, no different from statements about the future. In BOTH cases we have no way to know, hence BOTH are indeterminate.