It sounds like you're describing an external staircase - loosely speaking a fire escape. I could be wrong though, I'm not sure.
The article was discussing the requirement for _fully enclosed_ internal staircases, which is different.
These also have the added bonus of being generally a nicer staircase than the one that's actually built into the house. The interior, retrofitted staircases tend to be narrow with lots of awkward, angled steps.
Per the article, the latest IBC codes seem to say each stairwell must be fully protected/enclosed and the old external ones are insufficient. I used to live in a building with something similar and if yours is like mine then I doubt it meets the latest 2nd stairwell requirements.
(Which is a shame since I agree those backyard stairwells are lovely, and landlords will be even less likely to build them if they have to build 2 internal ones already. Of course if those stairwells increase property value/rent revenue, then that could incentivize them anyway)