One missing principle is Hike Your Own Hike. We can follow our own drummer! I'm sure many hikers adjust their gear list to their own comfort and pleasure. Some use UL techniques so that they can take photo or video gear, climbing equipment, fishing gear, pack rafts, etc.
One of the core UL principles is that you have a choice about your pack weight, that it is under your control. The first application of that concept is that you don't need to haul a 50 pound load to be comfortable and safe in the backcountry.
The other side of that coin is that you don't need to be miserable due poor clothing choices, uncomfortable sleeping pads or the like. You can use some of those saved ounces for more comfort if that is your choice.
One way to look at it is the application of the UL concept that you take only what you use. If you use your book, journal, or camera, so be it. You can still seek out the lightest, highest performance versions.
The idea of "luxury" items gets a little Puritan at times. To get UL you need to be able to grasp the difference between essentials and other gear, but the idea that you can never take these items is too conformist or orthodox for me. I'm happy to have a light load with my essentials to allow adding a camera or a book and still have a reasonable pack weight while knowing the difference.