THere wouldn't be TRIAL evidence that it was true, because the stipulation would be the only evidence necessary, and any attempt to introduce other evidence about it would be rejected as not germaine because of the stipulation.
Sometimes defence will stipulate because they fear that the prosecution's introduction of the evidence to prove a point will be more damaging to their client than simply conceding the point. Of course, if the PROSECUTION feels that way, they might oppose stipulation -- but a judge may accept the stipulation anyway.
The lawyer could have been incompetent, but if her experts told her there was no disputing the bullet analysis, she probably figured that fighting the idea that they bullet was Ramos's bullet was counter to her defense strategy that the shooting was justified.
If you think the shooting is justified, and you can't deny that you TOOK the shot, what's the point of getting into a losing fight over whether the shot hit the guy? It will just look like you think it would be better had you not shot at the guy to begin with.