IMO that phrase came about when old tech companies (the IBMs of the world) had

  * waterfall
  * design up-front
  * source control systems that
    * defaulted all files to read-only
    * required you to "check-out" files, potentially locking other devs out from editing them [1]
  * probably didn't have unit tests so "deploying to prod" meant "doing a full QA pass, done by human beings"
  * there was no CI/CD (We had "Build Engineers")

In this context, pushing a change to SVN/git/hg, having tests run automatically, then having CI/CD push new code to production, all as a side-effect of one engineer push a button? That was moving fast, and occasionally, breaking the whole website. But we got better tests, better CI/CD, metrics, green/blue, ... We learned it was unequivocally better than the old way.

[1] Reserved Checkouts: https://www.ibm.com/docs/en/clearcase/11.0.0?topic=ucm-check...