Cognac is high quality wine that has been distilled into liquor and put into usually a charred barrel and aged then bottled.
The fact it very old doesn't mean it tastes good.
It might this old because another bottle of out of the same barrel tasted like crap and nobody wanted to open the other bottle. So it sat on the shelf for over 250yrs.
I was going to say this.
The other non=alcoholic parts will turn bitter, precipitate out and the esters will turn rancid but the high alcohol content will prevent it from turning to wine.
My guess is it tastes like a tongue depresser dipped in alcohol and then rubbed with a thin mixture of tar.