"first established their presence" - "their" referring to clan Kragen, not the entirety of the Dwemer, and "first established", as opposed to "established their first presence", referring to the first thing they did in a series of many rather than it being the first presence. Although it was the first presence of clan Kragen. So there were Dwemer there before, just not those of clan Kragen. Interpreting it this way removes the contradiction you refer to and requires no further assumptions or speculations. We know that at least Mzulft and Kagrenzel existed in Skyrim long before clan Kragen came, with the Ayleid's launching an assault on Mzulft to retrieve the Wrathstone, which failed and lead to the construction of the Vault of Mhuvnak. Long after Mzulft fell into the hands of the Nords and was later retaken by the Dwemer around the time clan Kragen made its way to Western Skyrim where they formed an alliance with the already existing Dwemer cities of Raldbthar, the reconquered Mzulft and Bthar-zel.[...]the Dwarves of Kragen reached what we now consider Skyrim and first established their presence in the region with the founding of Arkngthamz.[...]