If you ever put "you just need" into a sentence about security you've already lost.
User input need to be sanitized always. Rails make it easy and is the default way.
Regardless, it's not a knock on rails. Just saying it's not necessarily a magic bullet to secure apps (and I don't believe rails nor PHP needs to be).
Edit: It's not actually a footnote -- it's actually just the only item on the lonesome "Security" page.
Which is why you should never process raw input in multiple places in your app. One method I've used for over a decade of webdev in any language: there's only one place to get the input from, and that place requires you specify what input is allowed.
While this may be not as easy to use as simple strings, it is consistent.