Your instincts are good. It's very much BS. They're the exact same thing except there's more liquid in the liquid type, and the way they process the dry involves vacuums and high tech stuff.
But really, some big brewery just makes a mash and collects the runnings and then concentrates it in one way or another. You don't have to boil longer, and doing so wouldn't break down sugars anyway...just make more unfermentable caramelization.
As an aside, I read the other day the scientific explanation for why liquid extract continues to darken in the can, which I won't bother to bore everyone with here. Basically, in the presence of liquid, the extract will always continue to darken, so liquid is less appropriate for long storage than dry, and you want to be careful to get the freshest liquid extract you can.
At the risk of sounding like a broken record, I just prefer dry. I think it's more flexible and gives you a cleaner base flavor to build on. Cheers!