How is KC an Augustine?
The writers have said that they like to think of KC as part of all 3 founding families but KC is not an Augustine. Kat being married to Elliot and KC forming a bond with him would not make her an Augustine.
Maybe the writers meant it in a metaphorical way, like KC is the one that unites all the founding families together but that is not true for the Augustine side. Kat and Elliot would have ended up together whether Jacob and Abby got together or not. So the quote in my opinion is quite misleading.