She chose to say in oral evidence that in her opinion (unqualified) it looked like insulin poisoning. I doubt this was a hugely influential factor for the jury though.
I think if I had been on the jury, the fact that even the defendant agreed that someone poisoned those babies with insulin would have influenced my opinion when set against all the other evidence.
If she had said she didn't know whether the bags had been tampered with because she didn't do it, then that would make sense. But agreeing that yes, it was attempted murder, but no it wasn't her didn't look great when the prosecution's case was obviously that she was the only one who had the opportunity to tamper with the bags.
Conceding that someone was actually going around poisoning babies completely changes the defence narrative that 'these were all natural but unexplained deaths and it was just coincidence that Letby happened to be on duty' - if I were a juror, knowing that these two collapses were accepted by Letby as deliberate poisonings would have influenced my view of her guilt when she was the only person who had the opportunity to do it.
So now, as a juror, I would need to consider whether I believe Letby's argument that not only did all the deceased babies die of natural causes, but also that there is an unknown person who deliberately poisoned several babies who is still completely unidentified (even by Letby, who was also present at the time of these incidents that she accepts occurred), despite several years of police and hospital investigations into this very issue.
That's a much tougher scenario to accept than mere natural causes and coincidence, but it's what the jury would have needed to accept as what probably happened to believe Letby's version of events once she accepted that some babies had been deliberately poisoned on her shifts.