That is correct, you can carry whatever coverage you have at retirement into retirement. So if you have option C, you can continue with it or drop down to B or even lesser coverage. If you have option B, you can't add C, but you can continue with B or lesser.
So the open season would be awesome if you were looking to add coverage and are within a few years from retirement.
But for most Mustachians this doesn't apply, as most of us aren't going to make it to the MRA. And you'd still have to actually think there's value in what the FEGLI offers, which it seems many believe is overpriced.