"[...] he made around $400/month when he started as a tester in 2015. In 2018, as a junior programmer, he said he was making ~$700/month" Take this in context.

"[...] he made around $400/month when he started as a tester in 2015. In 2018, as a junior programmer, he said he was making ~$700/month" Take this in context. $400 for a starting tester (aka zero qualifications) is standard for eastern europe. As a career, it's on par with call center operator. 2-3 years later, he switched to "junior developer" in the same company. I'm reading this to mean there was a conversation with the management that went like this: "You seem like a smart guy and you did a couple of javascript online classes. We'll switch you to dev and teach you how to program properly - oh, and you get 10% raise along with it. When you can do useful work we can renegotiate". One may teach himself useful programming in a couple of years while working full time as a tester - it's definitely possible. But the baseline probability is that at that stage he wasn't yet productive, and still required what's basically paid training for 3 to 6 months.