I see what you're saying, but the jury heard the parents' evidence as agreed facts (i.e. evidence that Letby accepts as true) when they were read out to them (with Letby sat there listening too), and were then blindsided by Letby suddenly suggesting n her cross examination that the parents' accounts weren't true after all.
Because the parent's' testimony had been agreed by Letby before and during the trial up until that point, the jury didn't have the benefit of the parents having being cross-examined on those points (and yes, I absolutely appreciate the optics of that and why it was preferable to spare the parents, regardless of Letby's guilt or innocence).
So while it might have been perfectly plausible that the parents had misremembered something or had mistaken which nurse called them, etc, the jury never got a chance to hear that evidence as the defence agreed their statements and so didn't need to cross-examine them and ask the question.
Therefore, the jury didn't have any idea what the parents' answer would have been to the question "could you have been mistaken, given everything that was going on at the time?", as it was never asked. It may be that a parent would have answered "yes, I suppose I might have been mistaken", but they might equally have answered with "no, it was definitely Letby because of x reason".
The jury just don't know, and so they couldn't just make up a conversation in their head of what they thought might happen if the parents had been cross--examined, and then pretend it's actual evidence of what that parent said - the parent was never asked in the trial, so that evidence of their answer didn't exist for the jury to consider.