Maybe, maybe not. Were it not for the "bridge" area it looks like the channel would support some vigorous currents (look at how blue the water is on either side of the bay) -- which possibly argues against the natural formation of such a sandbar.
I note that 17,500 years ago was during the last ice age -- ocean levels would have been lower, so if there was a low ridgeline along that route, it might have allowed people to move on foot between the two areas. No need for a manmade bridge then, and it would explain how a sandbar might occur naturally.