I think you misunderstood the question here. if the lenght of the ORF, not the AA sequence, is larger, not smaller, than the threshold, include it in the list. 

see this: 

    for ORF in all_ORFs:
        if len(ORF) >=threshold:
            amin = coding_strand_to_AA(ORF)
            threshold_ORFs.append(amin)