I know. I learned that not so long ago. But in present day nobody uses it in that sense. When was the last time you heard somebody refer to China as a Second World country? And people would think you are sarcastic if you called Argentina Forst World. Not to mention that the framing is US-centric. Why not the USSR First and NATO second?

In present day "First world" is used as "model for what a country should aspire to be" and "third world" is used as "failed to achieve that model" or even more despectively "third category".