What's the big deal with a 234 year old bottle of cognac? It's my understanding that distilled liquor is "dead", i.e. unlike wine or beer it has no inherent chemical change, merely the reaction with the barrel in which it is aged, so that bottling in glass (which is non-reactive) stops the aging process. Because of this, hard liquor that was bottled last week after 12 years in the barrel is "older" than some that was bottled 50 years ago after 4 years in the barrel.
You are correct with that. The only aging that occurs is while it's in the barrel(s). During that time it penetrates the wood and transfers the flavourants of the wood into the alcohol. Once bottled, there's no more flavourants to transfer so it's just sitting there. There's actually standards on how to tell how long it was aging:
VS= Very Superior - aged 2 1/2 to 4 1/2 years
VSOP = Very Superior Old Pale - 4 1/2 to 6 1/2 years
XO = eXtra Old - more than 6 1/2 years
So when buying cognac that's what you want to look for, not the made at date.
All that said, a 234 year old bottle of cognac would be EXTREMELY rare for simply surviving so long without being opened. Hence the price. Though if I were a booze aficionado (no, I'm not) I would go for a 12 year old XO over a 200 year old VS.