Sutta is a tool to guide us to the truth. It is simply a tool, it is not the truth. Since it is a tool and not the truth, it can be fabricated. The persons or details in the Sutta may be real or not real. However, that is not the purpose or concern of the Sutta. It does not try to convey the actuality of the events or persons in it. It is trying to convey some meanings that guide us to the truth.
If we look at MN91, it states that the marks of a great man has been handed down in the brahmin’s tradition’s hymns. It means that a great man is defined or determined by his appearance or his body according to this tradition. However, the Buddha never teaches us that.
Using the idea of the physical marks of a great man in that tradition, the authors of the sutta intentionally make the marks as weird as possible. This shows us the weirdness of the idea. Nobody who has not lost his mind can see that as normal or acceptable. That is what the Sutta is trying to convey. It also emphasizes the absurdity of the idea by adding the two parts that are missing and that the Buddha also wants to show off them.
By rejecting the physical marks of a great man in that tradition, it starts showing us the characters of the Buddha. His right mindfulness in all actions, his contentment, his right speech, right action… This is the true marks of a great man according to Buddhism. This is the real focus of the sutta.
In the next Sutta MN92, we can see the authors begin to describe how the Buddha looks like physically in its verse. There is nothing abnormal with the appearance.
I think MN91 is a very interesting Sutta. It helps us to distinguish the finger and the moon, and to break our habits of grasping onto the wordings or the persons in the Suttas.