There was some presenc before (in the 10th century) but after that you have to wait until the 14th century before another stronghold is built there.
I didn't see anything in the 13th century. Since the territory at the time was nominally Cuman, who are nomads, there wasn't much emphasis on strongholds.
And we can always play a little with history. There's so little info on the island that it's really not hard to do that!
Now, as for being situated a bit too much to the west...
You may be able to get there fairly quickly (1-2 weeks by ship), but how do you get back? No mundane ship will wait for you there, and they can't go upriver on the Diepner (there are 7 areas with rapids between the coast and Kiev, not to mentions the swamps). If you have to wait for weeks before another ship big enough to transport you, your party and your loot, wouldn't you rather do it in a covenant with a lab and a library? For that matter, you can plan you're next expedition from there and save a month's travel time.
While I agree that Novgorod may have little interest in the area per see, setting up a covenant there to act as a base camp allows the tribunal to get the benefits of exploration in the area while still claiming the territory itself (and thus preventing others from exploiting it). The certainly wouldn't want the Rhine tribunal trying to claim it (there's a lot of enmity between the two tribunals). To a lesser extent, that's can also be true with Thebes and Transylvanian tribunals, I believe.
I simply need to know. Are we taking this location or not? I think we can have it make sense, and build the stories accordingly. But if people prefer that I move thing east, I can do that. But I looked a bit at the north side of the Sea of Azov, and the land didn't look promising there. And it's deep in Cuman lands and very far from any support structure -- you're adding weeks of travel to get there, and don't gain much more variety to the territory to be explored.
I just need to know before I invest more time in the proposed location by building up the magical aura, rationale for it, local secrets, resources, etc.