I agree with a lot of your points, but I disagree that Fed's game has not improved, especially his backhand. I think the standard is higher today ... baseline bashing is a huge part of the game and Nadal, Djokovic, and Murray have raised the bar. His backhand has become better if for no other reason than to compete with these guys.
But there are intangibles, of course. Confidence, movement, fitness, etc. At his prime, Federer had more of each, and I think maybe that is what he's referring to. In terms of the stroke it self ... the consistency, power, and even spin I would say is better today.
I think in some respects Federer has improved and in other respects he has lost something, as will happen with age. I think much of his game has just been maintained at a high enough level to remain competitive and adapted to current circumstances.
There are human limitations. Once you get to such a high level in any physical activity, you will reach a point when the learning curve flattens, at which point your level will fluctuate above and below that line from one day to the next, but will not improve dramatically. I don’t think you perpetually improve when you are already almost at your limit or the current human limit.
I think now he is a physically stronger and more experienced player, which will manifest itself in a variety of ways on court. I also think the power he is capable of has been maintained from his younger years, but he does not appear to employ it as much. Nevertheless, I think that tactically he is smarter.
What he has lost are the intangibles you mention, which in many respects are the most important thing at these high levels. He is not as quick and sharp as he was at his best. This is important because, as any coach will tell you, getting to and reacting to the ball in time is half the shot and will improve consistency significantly. Then there is the confidence and the focus/hunger, which for most champions peak when they are in the prime of their youth, before doubt and fatigue start to emerge. Federer is not as fresh as when he was younger and cannot only focus on tennis now with a young family. And regarding fitness, it becomes more difficult to recover from exertion as you age.
Aside from his movement, the thing I have noticed most about the current Federer compared to his prime is that he makes more unforced errors than he used to during his best years prior to 2008. In 2008 his unforced errors really went up, which has improved since then, but not prior to that period. Obviously he had health and fitness issues in 2008, but I don’t think he has ever been able to get back to that bubble of confidence or mental and physical synergy he had at the peak of his powers. There was a time when his game functioned more like clockwork, particularly 2004 – 2006, when you knew even before the shot, that a short ball meant the point was over. Those days are gone. Now he has many more unprovoked ups and downs in his performances.