The meaning of any utterance can only be understood through implicit or explicit interpretation. A prosecutor trying to prove that Trump incited an insurrection must, as hszmv layed out, show
- His message was a call to insurrection.
- He knew what could happen.
- He did it intentionally.
I'll focus on the first requirement because you ask specifically about whether ambiguous language can prevent the prosecution from proving that he meant that.
The question concerns the core of all communication: How do we know the meaning of something? The most basic theory, a mere description in fact, lists three steps of information transmission:
We can assume that the transmission and the physical level of encoding and decoding was near-perfect; there was a sound system and there are recordings, so there is not much ambiguity on actual signal. The prosecution must then show that Trumps ambiguous wording was an encoded incitement of insurrection. They must "reverse-engineer" the encoding Trump did, that is, decode it from Trump's perspective.
As an example, take the sentence "Give him the special treatment." Obviously a waiter in a bar whose boss just pointed out a celebrity would not go and shoot the patron. On the other hand, a gang of Mafia henchmen would not go and give their victim the best table in the house after hearing this from their boss. The respective recipients properly decode it because they consider the context. Natural language is imbued with meaning by context. (As an aside, that's why it is hard to understand for artificial intelligence1.)
Examples for context are
- The position or function of the speaker (Bar owner or Mafia boss)
- The audience (a waiter or a group of armed killers)
- Other utterings preceding or succeeding the one in question ("he was at SNL yesterday" vs. "he didn't pay yesterday although Enzo asked him very kindly")
- The setting (a restaurant or a Mafia home)
- How past utterances in similar context were meant.
A court needs to — implicitly or explicitly — reproduce this encoding considering the relevant context.
Whether you like it or not, to answer the question we necessarily must investigate the intended meaning of this particular use of "fight"; since you are not interested in the details I have moved it to the footnotes.2 But it is for first reasons unavoidable that prosecution, defense and court each must perform an interpretation — namely the attempt to reproduce the speakers "encoding process" — of the spoken words.
1 As a famous urban myth example: "The spirit is willing, but the flesh is weak" was translated by an AI as "the liqueur went down nicely but the steak was bad."
2 Yes, fight can have different meanings in different circumstances. If you make an appeal to "join ChildFund in the fight against child mortality" no one will reach for their baseball bat. But Trump's use of "fight" was in a decidedly different context from which we can infer its meaning. The passage started with "we will never give up, we will never concede" (emphasis mine). "Concede" has a specific meaning in an election context: It means that one accepts the result of the election and adjusts one's behavior accordingly (prepares to move out of the office, starts the transition of command etc.). At this point the election result had been confirmed by the relevant administrations and courts. The statement "we will never concede" under these circumstances is the refusal to acknowledge the outcome of a legal procedure. It is a resolution to be lawless.
This is the stage that was set for his infamous "If you don't fight like hell you're not going to have a country anymore". The context is a commitment and appeal to lawlessness; the audience is not a convention of lawyers who would fight in court; it is not an assembly of campaign workers who go out and fight for every vote; this is a partially armed crowd of vigilantes. "Fight" in this context must be understood as a physical, violent conflict.
This appeal passes legal muster which renders it unprotected by the first amendment:
- The action is imminent: The speech is held in the vicinity of the Capitol, with a crowd ready to take action.
- The action called for is unlawful: As we established, he calls for a violent intervention.
- Trump has mens rea: He openly refuses to lawfully concede. The call for violent intervention is a call to help him resist his removal from office.
His speech is a conscious appeal for an imminent, unlawful, potentially armed intervention against the final outcome of an election. It is an incitement of insurrection.