Feedback is important, so after the interview, everyone gets detailed personal feedback on every section, regardless of the outcome. We get much higher resolution data after the interview (part of the point of my article).
You may have hit a few React-specific questions, but getting those right is certainly not required.
As far as Boston/remote: interviews are expensive for us, so it makes sense to interview engineers who are excited about working in places where lots and lots of our companies are hiring today. Currently that's SF or NYC. When we expand, you'll be able to pick up from that step.