Disclaimer: I just graduated last week. I think the stated goal of the program is to make you a well positioned beginner. So you're right in saying that there's no way for DBC to offer the depth or span of knowledge that a 4 year CS degree offers, but that's not its goal. I think the program is designed to teach you how to learn about programming, and give you a huge jumpstart in doing it. Consequently, nobody there is religious about any one topic. Yes, we all learn Ruby, Rails and Javascript, but what we really take away is not the knowledge of how to do things in Rails, but the ability to learn programming concepts, now and in the future.
As for learning on your own, I would respectfully submit that learning often happens in groups. At DBC there was a lot of solo time, but there was a lot of pair/group time, where we continued learning from each other. Group time is arguably more valuable since it not only affords the opportunity to learn form what you're doing, but also what everyone else is doing, to say nothing of the meta-skills gained from working on software in a team. I personally gained a lot being immersed in an environment where 40 other people were learning with me versus trying to do it on my own.
You're right: I think most people used jQuery for ajax. That said, if Marty McFly went back and destroyed jQuery one day, I'm sure it would only take about 5 extra minutes for anyone to implement XMLHttpRequest instead. We all had to figure out how jQuery ajax calls worked, we could figure out XMLHttpRequest as well.
Finally, thank you for keeping an open mind despite your skepticism. So many seem to use any skepticism to completely dismiss the concept. There are also many on here who support it, but it's a pretty open pace so I'd suggest you go check it out for yourself during the Fall cohort if you're curious.