So, this person was working basically non-stop for 3 months to make 65k. Working 12 hours a day for 90 days would yield an hourly rate of roughly 60 USD. Not sure this is sustainable or worth it in the long run.
To give additional context, I worked non-stop for one month to make the app
Other 2 months were pretty chill, I did marketing mainly, which didn't took all my time :)
But before that, I worked non-stop for 6 months or so, I learned to code in May and was learning + making apps to learn non-stop from May to January
Assuming a 4% safe withdrawal rate, 65k gives you 2600 USD an year. Moving to a country where that is enough to retire on will mean a significant reduction in standard of living.