Mount Targon has been shown with their worship of the Sun, as shown with Leona. They believe that the Sun is an entity, whose power is great.
Shurima's lore tells us the power of the Sun Disc and their worship of it.
So are the followers of Mount Targon the remnants of the Shurima Culture, those who might've survived the fall of Shurima? Not the current followers, but the people who started it all.
I don't think there is a relation between the two cultures. There were a few oceans between the Aztecs, Japan, China and Rome and they all have sun gods and goddesses. Even the Judeo-Christian religion has a basis in sun worship. It's a universal constant that ancient religions worship the sun. The Sun provides light and heat for life to grow. In my eyes, it's just a coincidence that both Targon and 
SHURIMA happen to worship the sun, but as a writer, I know that in fiction there are rarely any coincidences...
I wonder what does Leona think about it, if she knows what Shurima's sun disc is (or what she would've thought, if she doesn't know about Shurima's sun disc), and the opposite (what will Shuriman people think about her).
I have a bit of head canon that Shurima and Targon were once sister trade cities with a similar/intermingled worship of the sun. Targon was a place of pilgrimage being the tallest and thus closest place to the sun. Shurmia in turn was the true trade hub being a desert oasis at the center of southern Valoran. This is mixed with my other head canon that Kalamanda is a secret underground city that is at the base of Targon that provides passage between and to other places to those in the know cough Taric cough.
I think there's potential for awesome interactions between those two factions, the precedent being the real-life relations between Rome (Targon is greco-roman inspired) and Egypt (Shurimas inspiration) by way of Caesar and Cleopatra. It's exciting to theorize how something similar could translate into Valroan's setting!
