I think because on a fundamental level, most people really don't understand someone confessing to something they didn't do, or confessing to something they did do, but including details or creating a narrative that doesn't fit the evidence.
Not implying anything about the defendant's guilt, innocence, or guilty knowledge in this case, just talking in general terms.
There's just always a suspicion in people's minds when there's some kind of disclosure, even if the content is provably false - for example, the perp was in prison when the crime happened and could have had nothing to do with it.
So from a defensive position, starting from a point where their client has made some kind of admission is a harder one than if they said nothing at all. Even if the other facts of the case are identical. So I can understand why they want the confessions either not admissible or debunked by their 'confession expert'. It gives them an easier time of things defending him if the confessions are off the table or devalued in the eyes of the jury. It's probably one of the few things I actually understand about the defence strategy in this case to date.
I am not a lawyer, so MOO.
You're not a lawyer, but you might be better than this defense counsel. I kid, I kid! I think your analysis was spot on.