Maybe because it's based on gravity?? [$]O(1/r^2)[$] interaction.The same idea as in economy, but here applied to the flows of people between places. In Ferguson's model the probability of transmitting the virus between two individuals is generated by kernel functions, whose values decrease with the distance between those individuals. I'm not sure why they call it a gravity model, but ok.
And it is indeed mentioned in the code.
* @brief Holds microcells.
* Keeps track of susceptible, latent and infected people (in addition to details like who
* is vaccinated, treated etc.) Also contains data for the spatial gravity model for social
* interactions (probability distributions).