But does anything above 100% not give a false picture of the image? I get jaggies with full size RAW - at least on some lines
That's my point. It depends on the original image size.
Your eyes can only discern a certain amount of pixels per inch. Anything more becomes
indiscernable. Anything less and you potentially start to see the pixels or their shape. Assuming 20-20 vision, you can't resolve better than 300ppi at 12", 115ppi at 30" and 50ppi at 6 feet (TV distance). With Blurb, and thus a book, one could assume 300ppi (a 12" reading distance). So if you take your image dimensions in pixels and divide by 300, that's the physical inch size it can be and you won't see anything you can't already see at a normal viewing size.
Jaggies (if i understand right) are artifacts generally down the edges of contrast in an image and are often seen when zooming right in. Lets assume you have an older camera, or a crop, (or both) and your image is 1800 x 1200 px. Dividing that by 300 gives me 6x4. As long as I print to 6x4 or smaller, it will look like it does on the screen - especially if I can arange the image on the screen to present an image that is 6x4 inches in physical size. Even if I slap it into their biggest book (12"x12") it will be 150ppi which will look fine to my (old, non 20/20) eyes.
Viewing at 100% or 200% or 500% doesn't really help. You need an idea of what size it's going to be displayed at to understand how "miniaturised" the imperfections will be. Instagram for example is an online only thing. On my phone, the image size will be roughly 2" square. On my desktop it's about 5" square. The quality is about the same because the phone is often closer to my face than my monitor.
I think this is exactly where the information is unhelpful. People who know no better will be examinig their digital SLR files and seeing imperfections at 200% but there's no way they'll see them in print. However there will be people trying to print heavy crops from a phone camera which will also look bad and they will be seen in print.
It's generic information designed to prevent complaints. It's similar to how print companies request 300ppi no matter what size you're trying to print to.