Exactly, it's kind of back to front. "Look at all the babies who died when Letby was on duty, see if any other nurse was on duty for every one - no, only Letby". However, if you included deaths where she wasn't on duty, you might have found another nurse's name who was present for (say) 5 of the deaths attributed to Letby, but who was also on duty for some of the other deaths.
I'm not saying the conclusions were wrong, but if this was in fact the way evidence was presented, it's very clearly inaccurate.
edited to add - it may be that they only looked at unexpected and unexplained deaths. In a neonatal unit it's an unfortunate fact that babies die, due to prematurity, respiratory problems, infection etc.