No, it's not. The idea is that your should be able to (hypothetically) open source your app at any time without compromising any sensitive information.
Figaro is certainly usable without Heroku. It added some Heroku conveniences later in life, but is built to simply populate ENV while keeping source control clean.
Not quite because the summary could change to nil, but it brings up a good point. There's often a better way in Rails, but &&= is native Ruby so it's especially useful in non-Rails applications.