NASA's DART impactor made a head-on collision with the asteroid Dimorphos on September 26, 2022. A real-time video feed gave immediate confirmation of the direct hit. But according to this press release, NASA had to observe Dimorphos for two more weeks before being able to confirm that Dimorphos's trajectory was indeed noticeably altered (as planned).
Why? It seems to me that determining the collision's effect on Dimorphos's orbit would be a very simple exercise in Newtonian mechanics. I assume that Dimorphos's total mass was well-known from its orbital dynamics with Didymos. I know that its internal composition wasn't well understood, but is that really so important for understanding its post-collision dynamics? Conservation of momentum means that the subsequent overall motion of Dimorphos's center of mass should not be affected by the details of its internal composition.
I know that the collision ejected some material off of Dimorphos's surface, so there's a bit of a semantic question as to whether after the collision, the term "Dimorphos" should refer to "all of that material that made up Dimorphos before the collision" or "what's left on the largest connected component of that material after the collision". But it doesn't seem to me that this would make a big difference regarding Dimorphos's overall dynamics. It seems to me that approximating the collision as a perfectly inelastic collision between two point particles would probably give a pretty good model. Even if the impactor did knock off a significant fraction of Dimorphos's mass (which seems unlikely), then it seems to me that this outcome would count as "significantly changing its trajectory" almost by definition.
Was there ever really any genuine uncertainty whether DART would redirect Dimorphos given that DART directly impacted Dimorphos? What kind of plausible internal composition of Dimorphos could have led to a failure to be redirected?
Edit to clarify question scope: As is often the case, many people are interpreting the title of my question too literally. (My understanding is that Stack Exchange's convention is that the "official" version of an SE question is found in the question body, and the purpose of the question's title is to draw attention rather to precisely state the question.) I'm not trying to have a general philosophical debate about how much you should trust theory vs. experiment. Nor am I trying to understand why NASA actually did observationally confirm the redirection, as a lot of complicated non-physics factors enter into that decision. (So any speculation about NASA's political incentives, etc. are out of scope for this question.) I'm just asking, very concretely, what were the main sources of scientific uncertainty in the extent to which Dimorphos would be redirected given a successful collision, and how those uncertainties would affect the extent of redirection. "The composition of Dimorphos" would not be a concrete enough uncertainty; I'd like to know how the composition of Dimorphos would change the redirection. Of the many comments and answer to this question so far, only John Doty's answer addresses my question within the scope that I intended it.