As someone who lives in a real third-world country...I'm of the opinion that this is a silly thing to say. Yeah, the world's biggest economy ($23T GDP) that constantly sends payloads to outer space and back is third-world...maybe you could come live in a place with 80% of people below the poverty line and see what third-world is.
When a country’s poor can still manage to be overweight, have cell phones, have gaming consoles, have automobiles…your perception of poor might be a tad subjective.
And before folks go off on the “what abouts”, yes I realize there are some folks in the US without those things, but let’s face it, you sample just about any “poor” person in the US, you will find at least 50%+ of the things I listed.
I doubt that you could do that with a random sample of the poor in say Burundi
The sign of a first world country isn't having a bunch of billionaires - you have those in Brazil and Russia. It's when the poorest people in your country still can have healthy, meaningful lives.
Honestly, from my point of view, the US is a 3rd world country, but I expect that viewpoint to be very controversial.