He was refering to some older stages of the file from around version 4.4 where you could find /** Defines a triangle by its vertices. */ virtual void DefineTriangle(const TStaticArray<FPrimitiveTriangleVertex,3>& InVertices,const UMaterialInterface* Material) = 0; But seems like they changed some stuff in the file.
You can just check the old code on github if you linked your account.