Not sure how fate would be considered more complete.
So, I think the definition of "completeness" being presented is equivalent to:
"A game is complete when there are no legal action declarations the player can make that are not covered in the rules."
Chess, for example, is complete, because the legal moves for each piece are explicitly stated.
The suggestion is that Fate's mechanic is so generalized that every "legal" action declaration can be covered by them, while D&D has holes in it that the GM must patch over.
Ship to ship naval combat, for example, is not covered by the Basic rules of D&D. We are given no rules for how ships maneuver, but PC abilities rely on knowing map distances down to about 5' resolution. The GM must make something up if they want to run ship-to-ship combat, while in Fate they don't.