I very much doubt Letby has the resources to assemble a team of the world's leading experts during the trial, who could have covered off every postulated cause of death.
As you would know, if you read the reports, Dr Shoo Lee became involved after learning one of his papers was relied on during the conviction, and that the prosecution had misinterpreted his findings and, consequently, misadvised the court.
His specific expertise was with regards to air embolisms in infants and, after realizing that the prosecution's case (to the extent it was based on his research) was not only incorrect but medically impossible, he has seemingly assembled a team with wide-ranging medical expertise to review all of the medical evidence and they, together, have come to the conclusion that there is no medical evidence, whatsoever, that any of the babies were murdered.
I find it highly unlikely that so many leading, impartial experts have got it completely wrong but the far less qualified and seemingly partisan* medical expert (who relied on, but failed to properly understand, research from the leading experts) got it right.
- I say "seemingly partisan" because an experts' duty is supposed to be to the court but there were concerns in the Letby case that he failed to meet that duty and, in a separate (concurrent) case, he was eviscerated by a judge for his abysmal medical evidence and for acting in a highly partisan manner and failing to adhere to his duty to the court.
Honestly, I really did think that the "Free Lucy" stuff was nonsense but I'm now very inclined to think this a horrific miscarriage of justice.