They don't.
Competency in this case would be relevant to whether or not he can actually be considered to be fit to stand trial: i.e. is he able to understand the proceedings against him and assist in his own defense. If he is not, then typically the individual undergoes psychiatric treatment to attempt to restore them to a state of competency before they can be brought back to court and have a trial proceed.
But yet....they've had all this time to work on ascertaining this fact, to question it, to get it evaluated.
But only now, once it looks like every strategy they have is being ruled against, do they begin to float this as a potential issue. Despite claiming his behavior at arrest was abnormal.
Sure, Jan.