So, from reading it's clear that they are only just scratching the surface of the genetic correlation.
You are absolutely correct in that migraines are caused by many different factors, the glyaecemic connection they found is just one of them.

I know for me drinking water pretty much all day (not excessive amounts, but just being sure I'm drinking it all day long) has had a massive impact on reducing my migraines.

So ya, the article is only really talking about this food/blood sugar connection, and this is simply more data that will help those who suffer from migraines with those types of triggers.