I wrote this a long time ago, but I think the metaphor was about generative AI applications vs. traditional software applications, not about AI coding agents vs. writing code yourself.
Maybe the train is software that's built by SWEs (w/ or w/o AI help). Specifically built for going from A to B very fast. But not flexible, and takes a lot of effort to build and maintain.
> (The horse) is way slower and less reliable than a train but can go more places
What does the 'train' represent here?
A guess: perhaps off-the-shelf software? - rigid, but much faster if it goes where (/ does what) you want it to.