I've used plus ortliebs going back to 1993 or 94 and can also confirm that they are waterproof. While they do feel flimsy, I've been impressed by how tough this material is also.
When considering biking up to Scotland to see my family up near Inverness way, the one consolation was that all my ortlieb stuff, all, really is waterproof!
Tis a shame I won't be able to make it, but it is what it is.