The various calamities that will befall the people of Noah do not come simultaneously. Some come as the result of the armies of the Lamanites, some, such as the burdens on their backs, come in the aftermath of their subjugation by the Lamanites. All comes to pass, but at different times. These last conditions are those under which they were living when Ammon arrived, a scene we have already read, but which lies still in the future at this point in the flashback narrative.
Translation: Abinadi uses the phrase "they shall be driven before like a dumb ass." The understandable reference to this is that the people will be used as beasts of burden, which is the state in which Ammon finds Limhi and his people. The translation problem comes with both the animal and the conception. There are no asses in the Western Hemisphere prior to European contact, and there are no known beasts of burden. Thus both the animal and the concept of using an animal in this way would be foreign to Abinadi.
This is a case of translation of meaning rather than words. The modern reader clearly understands Abinadi, but might not understand a more culture bound phrase, such as "tributaries with a tumpline" (the tumpline is a native method of carrying heavy burdens). The translation of the text of the plates would, in this case, be one of translating a concept rather than the specific words used on the plates. This is consistent with the less-than-literal translation mode that has been proposed in this commentary.