Edward III's whole argument was that even though Salic law prevented a king's daughters from becoming the monarch, the son of one of those daughters could still become king. So Edward's claim is that he was the closest using that definition.
But he wasn't. Philip V had a daughter called Joan who married the duke of Burgundy, and had a son called Philip. Philip was born in 1323 so would have been alive and the first in line using Edward's definition of Salic law when the Capet line died out in 1328. Edward III would even go further down the line in subsequent years, by the time the actual war broke out in 1337, Louis X's daughter Joan of Navarre now had 2 sons, and Philip V's other daughter Margaret of Burgundy had 1 son.
Did the French ever take notice of this in order to discredit Edward's claim at all? (because it seems like something that would instantly make the English claim fall apart, yet I had never heard of this before at all) Louis X's grandsons even fought for the English at times during the war, I wonder if they realised that they were fighting for a definition of Salic Law that meant they were the true heirs to the throne?
Okay you confuse one or two things about what happened, but this is common.
Philip V became king after his brother Louis X le hutin (Ok Jean Ier is btw but he liver 4 days). But his brother had one daughter. Nonetheless we weren't sure if this daughter was truly his (tower Nestle Affair), this daughter Jeanne of Navarre was put aside in the succession as the say that no woman can inherit french crown. The daughter of Philip V who was also named Jeanne could not claim the throne since it was her own father who enacted this law.
Edward III claim wasn't denied because he was in matrineal line. Edward claim was denied because he wasn't born on the french soil, and barons had decided that "no foreigner could be king of France". It was to avoid any interest conflict. But also because Edward III was king of England and french did have to fight England in Flanders and Aquitaine, Gaucher de Châtillon was absolutly against. Gaucher was such an important baron (served 6 kings), that his wards against Edward III was suffisant to descredit him for good. Nonetheless Philip de Valois was considered as a way better candidate than Edward III. He was descendant of St Louis in a male line which was VERY important, St Louis had such a moral importance. He was an important baron, and Valois family was for nobility rights. During this gathering Philip of Valois was elected by the french most important lords as king of France This include the duke of Burgundy the father of Philip of Burgundy. By electing Philip of Valois he denied for his spouse and all his descendants any right to the throne. Doing otherwise would be a "parjure" which was a felony and permitted to the king to size his duchy (which oftenly happened with the support of the sized duchy vassals).