马 (horse) was the main transportation before cars.
A single horse was the most common personal ride. That's why a horse arrived also meant a person has arrived.
马到 --> 人到
马到成功 -->人一到就成功了 (success as soon as the person arrives)
That's why dictionaries translated it as '事情顺利，刚开始就取得成功' - being successful at the earliest stage of the engagement. It is ...
Your best shot was pretty good.
言不及义 - A person's talk does not touch/reach the true/real meaning of the subject he wanted to deliver. He either does not have a thorough understanding of the subject or is not able to organize his thought in a logical manner.