The article is not saying that the plague was not spread by rats and fleas but that it is unlikely that it could have spread as quickly or for as long as it did exclusively by the rat/flea vector.
It suggest that the plague was spread by human to human contact and that the rats were a supplemental spreader.
This makes sense given at the time human fleas and lice were the norm. Everyone had them.
What I would like to have more info on is the idea that the soil conditions in Europe would not support plague bacteria survival.
This makes sense given at the time human fleas and lice were the norm. Everyone had them.