It's not extra work. The site in question was for making a reservation. A basic HTML for would have satisfied that requirement and worked completely without javascript. Any additional features could have been built with javascript and the site would be no different to users on modern browsers as the current build, but it would still work on the author's mom's browser.
Progressive enhancement isn't about building sites completely without JS. It's about building resilient sites that work on edge cases like this.
It is absolutely extra work. You have to test every mode of operation. So to support those with JS disabled (about 1.3% of users according to some quick searches) you have to test with JS disabled. Then you have to test again to make sure your JS workflow is stable.
Every business has to make the choice, but unless I was running a pretty large operation, I'd be willing to ignore 1.3% to put my budget into other things.
Progressive enhancement isn't about building sites completely without JS. It's about building resilient sites that work on edge cases like this.