Hi Oliver
I think such generic secondary sources as the Dictionnaire de la Noblesse, that don't usually cite primary sources are not suitable as sources for Pre-1500 profiles.
Charles Cawley's Medieval Lands database which is considered more reliable does have a section about the Seigneurs de Fiennes but for that branch of the family that became the du Bois family, he is relying solely on Pere
Anselme's Histoire généalogique et chronologique de la Maison Royale de France, and the Pre-1500 Resource Page does list this but has a note that it should be used with caution. Anselme's Genealogie of the Fiennes family is in volume 6, starting on p. 167.
Although the Charles de Fiennes is mentioned on p. 174, (though I suspect his birth date is before 1500, his parents were apparently married in 1480) there is no other information about his son Antoine de Fiennes, other than he was Seigneur de Vermeille, which makes the fact that he is listed on WikiTree with a wife and son, somewhat suspect.
Overall, I think we need better sources before adding the connection of Charles de Fiennes back to the earlier Fiennes family, and better sources for the Fiennes or du Bois genealogy after Charles.