From their abstract:
Our results support that human ectoparasites were primary vectors for plague [...]ultimately challenging the assumption that plague in Europe was predominantly spread by rats.
Basically, once there is an outbreak the speed of the outbreak can best be explained by a human-human parasite transfer. Human to rat to human would be too slow for the disease to have spread that quickly.
- But that's only half the story and the rats still play an important role:
They are the slow transfer and form the reservoir for the disease. Without a second, much slower transfer method, the plaque wouldn't have been able to cross oceans and jump from continent to continent.
People don't sleep with rats in the same bed, they don't cuddle them, share their icecream cone with them or kiss them, like I've seen them doing with dogs. Rats are therefor the perfect low infection risk reservoir. But once the number of humans increases, the total risk increases and an infection will occur and spread quickly through the faster method. Once the majority of the population is wiped out, the disease can travel with rats to a new place or stay in the rat reservoir till the population has rebound.