Because her defence team obviously hired their own medical experts, and if I had been on the jury, I would expect that if there were a plausible alternative scientific explanation to the one presented by the prosecution around the insulin, then the defence would have contested the insulin evidence and presented an alternative argument.
So if the defence simply agree that the insulin evidence is accurate, then the jury obviously have no reason to doubt that evidence and can just accept it as a fact.
While I think the insulin evidence (and the fact by accepting it Letby was effectively asking the jury to believe that someone poisoned babies on her shifts but it wasn't her, along with all the other purported coincidences) was probably pretty compelling to the jury, I doubt it would be grounds for an unfair trial, as the defence were presented with the evidence and had the time and resources to check it's validity before accepting it as agreed.
So although the defence might have made a strategic mistake in accepting it when they could have instead challenged it, and this might have changed the narrative they were asking the jury to believe, that was the strategy they chose to employ. AFAIK, Letby used the same counsel for the appeal and retrial, so it doesn't suggest that she thinks they fucked up - it looks like, for whatever reason, she accepted that evidence was accurate but also didn't for some reason suspect her colleague who was also present at both poisonings.
There were obviously numerous detailed conversations behind the scenes between Letby and her defence team around this issue (and presumably around who else could be to blame given they believed the evidence to be true), and what Letby and her team asserted at trial was the best argument they felt they could make for some reason, despite another nurse also being there both times.