The choice to use bootstrap is a common one, but is my no means required. It is part of the FCC curriculum, so it is a logical choice. But if you feel that there is a better library, you are welcome to use it. You are also welcome to use no library, but I think I would recommend using bootstrap and learn how to walk before you fly.
As to not using CSS, I think he was just showing how simple it could be. It was made easier because bootstrap comes with some styling. But if you want to use CSS in addition to bootstrap or without bootstrap, you are welcome to do that.
I think the spirit of Quincy’s example was, “Let’s keep it simple for the first project.” Some beginners get bogged down in obsessing over pixel perfection and adding superfluous features. In the beginning, I’d recommend shooting for simplicity, elegance, and basic functionality. As you get more experience, your instincts for what is possible and how much work it would take will improve.
The projects are not very strictly regulated. Meet the user stories, make something that works and looks good, living up to the spirit of the project. Within that you are allowed a great deal of latitude. If you aren’t sure, feel free to ask.