Earlier quoted context omitted.
New bar would still offer the old service (beer). Startup will offer new product/service.
A startup is a business with a high growth potential, otherwise it's just a business as already pointed out by @sillysaurus3. If tomorrow 250 people are queuing to get a drink from your bar, you won't be able to cater to their needs, nor the next day, opening a new brick-and-mortar business takes a lot of time. A startup would be able to scale (after a couple of days / week) and fulfill the customers' needs.
PG invented that definition when he started YC. It's a good definition, and useful in the VC world. But it's not the original definition, and most of the non-VC/Valley world still uses the original one: a business that is trying to build something new, whether it's aiming for big growth or just a steady new market.