The prosecution have established (and Letby agreed) that only there are only two people who had the relevant access to do it.
Letby denies it was her. Other option is not present in any other incident on the indictment.
Doesn’t rule them out though. As no tainted drips were found there’s no hard evidence of the means by which the insulin was administered or accessed, the assumption of how it was accessed is no more than that.