The video proves that feral cats in a colony of feral cats cooperate to defend the colony and therefore they are defending each other and their food source by chasing off a fox. The fox was perhaps looking for a meal.
In general foxes don’t attack adult domestic cats in my experience and opinion because they are too dangerous for a fox (in general they are equal predators) but they may attack infirm elderly cats and kittens.
However, despite the fact that cats are solitary hunters they do work together when living together to defend their community of cats. They are also probably intinctively defending their food source. This may be food provided by volunteers who care for the colony.
I found it an interesting video for that reason. It tells us that domestic, stray and feral cats are not entirely solitary in their behavior. In fact they can be quite sociable in various ways as can be seen in harmonious multi-cat households. Although there are many examples of friction between domestic cats in multi-cat homes.
In this video the cats were aggressive towards the fox. There was no standoff. There was no posturing and intimidation. It was straight aggression with the intention of forcing the fox to retreat and disappear.
