If you're an experienced developer, you should be able to pick up the static source code and figure it out how it was engineered. If you're not as experienced, you will learn through doing (not reading) and perhaps by working side-by-side with a more experienced developer.
If you're not a really good developer the question is how do you become one? It is possible to learn the hard way by doing (like you suggested), but it is painful. A playback is not just watching the code as it was developed but it is listening to the developer explain why they are doing things. My hypothesis is this will make it easier for people to become better.
People have a limited cognitive capacity.
There really may be some learnings worth extracting from a long-term source repository, but I think the cognitive overhead of looking at years of old commits competes with being able to focus on the code in front of you.
Then, when someone looks at some unfamiliar code they can search to see if there is a 'story' about that bit of code. The code and the narrative playback side by side. The noise can efficiently be bypassed.
