
The flashback hints at what probably happened during the void century.
D may not have been a clan or family name but a moniker that Joyboy gave to his allies or his pirate crew.
These people were strong willed enough to stand up to the 20 kings / the world government in the making and imagined a world of equality and liberty.
Just like Ace made Sabo a D. in the flashback, any strong willed person that opposes the world government and fights for freedom has what it takes to be a D. It's the belief in one's dreams and the will to achieve them that gives one the quality of a D, the initial D alone does not give a person these qualities!
This chapter's revelation absolutely DESTROYS Luffy haters who downplay his merits on a regular basis and credit his achievements, willpower and strength to Luffy's supposed special ancestry.
Tl;dr:
Luffy does not belong to any ultra-special god lineage nor is his family responsible for Luffy's willpower and his achievements.
GODfy top one all day every day