I hate how they worded the paragraph about impeding brain development. The goal of the research wasn't to create a disease model in rats, it was to create a more accurate replication of cell morphology than previous in vitro models could produce.

Cell development is incredibly complex, as cells respond not only to their own genome, but also to extracellular signals such as chemical signals and mechanical stress. Over at least the past decade, one way to account for this has been to shift from growing cells on a flat surface to three dimensional structures often referred to as spheroids or organoids.

However, organoids still aren't perfect representations of an in vivo environment. The importance of the difference in neurons post implantation is that their integration into a rat brain has allowed them to more accurately portray the differences between a healthy neuron and one with Timothy Syndrome than seen in organoids that remain in a dish. It wasn't that the Timothy Syndrome expressing cells were affecting the rat brain, it's that their implantation allowed them to display their own expected morphological changes.