Elturgard was formed in the 15th century. Elturel was older. Elturgard is formed when Elturel and it's Hellriders begin to take other towns and societies in the Heartlands under it's "guard", which starts
From your link:
In the late 15th century DR, it was the capital of the realm of Elturgard
By the late 1430s DR, through a variety of excuses, Elturel had laid claim to the lands of its neighbors, placing them within "Elturel's Guard" as they called it, making itself a petty regional power
That in turn only happened because of the fall of Scornubel as a major trading hub due to the Spellplague, which occured in 1385.
