I can totally see an umbrella for sun, that would be a no brainer. If it's flat, or rollling terrain. Or climbing mountains in the sun, for sure, I'd stick a trekking pole in the backpack for that, I hate getting heat stroke, plagues me.
I would definitely use an umbrella in the desert or high heat situations on more or less flat surfaces, but can't see using one in any other circumstance, though it would be fun to try just to see. But my experience with umbrellas has been that my pants and shoes get more wet than if I hadn't been using it, due to the streams of water coming off it.
However, blanket statements like, you don't need trekking poles if pack weighs < 10kg/22 pounds have precisely ZERO value, since there is no law or rule that dictates such things, it all comes down to weight on knees, condition of knees, age, and a variety of other factors, all of which vary individual to individual. When writing such an article, I suggest avoiding such fictional rules, which are literally made up out of thin air, and stick to facts and experience, which are interesting and informative to read. I can assure you that my knees let me know with zero pounds on my back plus descent, and have for years, but I don't make the error of assuming all knees will let people know this, although my suspicion is that many, if following the fictional rule of < 10kg on back, will find as they age that they really wish had avoided such behaviors. I have to be honest, and say I held off quite a while becoming a member here precisely because of such tendencies to just invent things and then state them as true.
However, more interesting, is rain gear.
I always scratch my head when I read about people's rain gear wetting out. I spent the worst winter in terms of rain in the early 80s being a bike messenger, using the old style heavy rain coats/rain pants, the canvas backed rubberized stuff, very airy, not tight or light, this was all day rain, every day, for weeks and weeks on end, rain so hard anything in your hand like a package would be soaked inside of a few seconds. I never wetted out, and I never got more than a bit damp, despite riding through the rain, in the rain, all day, each day.
So clearly there's something more along the lines of a rain gear fail going on with such rain coats/pants than an intrinsic problem with wearing raingear per se. And that is not hard to pin down, given this is backpacking light. This thread has made me accept the weight of my 12 oz rain coat, and I'll just happily continue to be a light, not ultralight, backpacker.