I have thought this too, that it could have been an accidental overdose. In the opening statements it seems they skimmed over the fact the baby had been prescribed insulin and it wasn't mentioned again. Hopefully more info comes to light on that prescription.
In regards to the breast milk and TPN... for many reasons, preterms need to build their tolerance to milk. Some can't start on 'full' feeds right away. So a tiny amount is introduced, as small as 0.5ml, and the rest of the required nutrition is given from TPN fluids. It's also important to start some milk to prime the gut, the smallest amount will do. It is my guess they started milk as soon as it became available, started basic fluids, then switched to the bespoke TPN when it arrived from pharmacy.
Insulin is always prescribed as a tiny dose for neonates... they're tiny after all. It is diluted into another fluid, usually saline. Any miscalculation in that preparation will lead to an overdose. The questions are, who prepared the infusion? And was the overdose accidental or deliberate...