Is Emacs truly the One True Editor?

Yes.

How much time does it take to master Emacs?

It’s a life-long journey.

What’s with the er- prefix in code examples?

Emacs doesn’t have namespaces and it’s a common convention to prefix functions to avoid potential name conflicts. er stands for Emacs Redux.

How to evaluate code snippets in Emacs?

Just copy the snippet to some Emacs Lisp source buffer, place your cursor inside the snippet and press C-M-x.

Alternative you can start an Elisp REPL using M-x ielm and paste the snippets there.