I think you basically have it right, but it may be worth considering how weight is carried separately from questions as to how your body can use water, and looking at the issues under a range of conditions.
At one extreme, if I’m walking along a stream and can drink whenever I want, it doesn’t make sense to carry any water, or to drink any more than what I need to stay refreshed and well hydrated. My hydration opportunities are already optimized, and anything I carry in my stomach or on my back is a waste.
At the other extreme, if I have 24 hours to go to my next water source, I might hope to drink 3-4L of water (YMMV) instead of carrying it. There are reasons why I might prefer to carry water in me rather than on my back (see below), but in this particular scenario, the strategy is going to fail miserably (even if I plan things so I won’t be walking too vigorously when my stomach is really full). Not having been made aware of my plan and the dry times ahead, my kidneys will do what they naturally do when faced with a lot of water – get rid of it. I’ll end my 24 hours feeling very parched. Much of that water was wasted.
In between, there may be some situations in which some waste of what looks to your kidneys like excess water is worth it if you get a compensatory advantage from using your body in place of a water bottle. To a first approximation, weight on your torso is weight on your torso (as opposed to weight on your feet), but all other things being equal, it might be slightly better to carry it in your body than on your back. It’s one less thing tugging on your shoulders, it’s one less water bottle to carry, and it’s one less thing adding to what your pack needs to be engineered to carry. These are all minor considerations, in my opinion, since your body can’t productively use enough chugged water to make much of a difference, but if you’re paying top dollar to shave grams from your gear list, it might well matter more than whether you treated yourself to the cuben or settled for sil.
The only time I really think about this stuff in the real world is when I’m heading out for 2-3 hours under circumstances when I can safely skip carrying anything at all if I skip the water bottle. A little water will go to waste, and I’ll be thirsty at the end, but I like the feeling of walking without anything on my back at all.