I think the reason why Agile worked, back in the days of the Manifesto authors, is not because of any specific thing in that Manifesto ... but because you had a small team that could decide how they wanted to work. This is antithetical to most corporate management.
Also presumably they were experienced engineers who could be trusted to make good decisions. But not all teams are like that. I have worked with many engineers who are completely lost, or will routinely make the worst decision possible, any time the requirements are exactly spelled out for them.
Little a agile was supposed to be an iterative system that the team could adopt and adapt to their needs. These forms were intended to make developers more effective.
The manifesto was so small and stark because those were the few things that all of the manifesto signatories could agree on. They were trying to find consensus on principles that made products successful.
Big A Agile is much more rigid and top-down, marketed to management. Success there means selling training and certifications. Nothing more.
Also, they were thought-leaders with an interest in exploring better ways of working, and presenting what they found. As such, they surely adapted themselves and bent some rules. They were not following recipes in some book about Agile. But if you look at agile coaches preaching on LinkedIn, there is an endless stream of posts about the true way to do agile.
Small teams can only get a small amount of work done. How long would a team of 7 take to deliver something like Word or GNU/linux (both not just the kernel, but everything involves in a OS/distribution )
