> Omitting the return statement is a good thing? How do you know if this method even returns anything?

All methods return something.

> If you're just scanning down the file, you can easily miss the if statement, and assume the do_something always goes off.

This is actually one of the arguable points in that styleguide. Some people share your disapproval for inline if modifiers. Anyway it's not that bad after getting used to as long as lines aren't too long and predicates aren't too complex.