It is a possibility, but we can't be sure without a close-up veterinary medical exam. Usually, bumblefoot is on the pad of the bottom of the foot and does not manifest itself by bulging at the top since the pad of the foot is more under the three digits (toes) instead of between the toes.

Another cause may be an infection if the bird has something impaled in a wound and tissue has encompassed the wound or the object.

In any case, as long as the bird is able to walk and access banks without any difficulty, it should not be a major concern. The Regal Swan