Earlier quoted context omitted.
But velocity of code written is not the same as velocity of value delivered. In Scrum, estimation meetings usually result in follow-ups with stakeholders that can (and often do) radically change the stories themselves. Totally different code winds up getting delivered that increases actual value delivered, even if "lines of code written" slows. That's the whole point of agile, to deliver value rather than code.
To be honest I have never seen this materialize. Usually you get some input from product management, the devs estimate it and then do it. I have never seen feedback from estimation to requirements.
> I have never seen feedback from estimation to requirements.
Perhaps you work with perfect pms! jokes aside, usually all it takes is an estimation of 8 or 13 and something changes real fast.