meaning of "this outlining performs best..."
It's the opposite of inlining, but not a standard term, which is why it is scarequoted.
why private outOfBoundsMsg is required
That's what "outlining" is about—extracting code to a separate method.
should I start refactoring my code to include string returning methods for my exception constructors?
If you care about 3 nanoseconds wasted each time you throw an exception which does not have a string literal for a message, then yes. In other words: NO.