Howard Roark is a static character in that he never "develops" by learning more about himself or conquering his inner obstacles to self realization or fulfillment. His overriding character traits are his unshakable belief in himself and his vision coupled with his rootedness in self-integrity and individualism.
When Roark turns down the lucrative contract, his reason is that the contract would require him to violate his architectural vision, which would be a betrayal of his personal integrity and an abandonment of his individuality. Therefore, his act was one of pure selfishness in that he was thinking only of himself with no thought to the broader picture or to connected concerns. Others could see it as "selfless" from their perspective because they didn't grasp the motivating force behind his actions.