If you look in the given translations for 'tombe' the second one is "falls off" so tombe + de can mean "falls off of".
Regardless, "he falls from his horse" and "he falls off of his horse" and "he falls off his horse" all mean the same thing. You can translate it however you like, that is half the joy of translating, you get discretion.
Grammatically, "son"/"sa" can both be his or her, but in context, the possessive term will be understood in reference to the subject of the sentence, i.e., he falls from his horse. If you need to clarify that she falls from HER horse, you need to add something, specifically.
- Il tombe de son cheval = He falls from his horse
- Elle tombe de son cheval = She falls from her horse
- Il tombe de son cheval à elle = He falls from her horse
- Elle tombe de son cheval à lui = She falls from his horse
I translated this as "IT falls from his horse", for example "The saddle fell from his horse." I was sure He falls from his horse was correct, but I wanted to find out if IT was correct too. Does anyone know? My answer was marked wrong, but I don't know if this was DL's mistake or mine.
Technically, we know that "il/elle" could be "it" referring to something previously mentioned. Without it, stay with the context. What context? Well, what is it that falls from a horse? A person. We can come up with a hypothesis that would justify saying "it" but we are not basing translations on hypotheses. We have precious little context but what we do have is sufficient to render a reasonable translation.
Two things to note Charles, every country has accents and dialects. Even in my tiny island of England, I can go not 30 miles away and not understand a word the person says. Secondly, Duo is a programmed computer course and will not have all nuances of accents. It is both free and has its flaws. Votre ami JJ.