I'm not quite sure what you said to him, but to tell him you earn a living as a writer on Medium would put you in the top 1% of Medium writers. If you earn $500 (or above) per month, you're in the top 3%.

You also only started writing on Medium in May this year, so I'm unsure why you would tell someone that. What did you actually say that received that kind of response. In reality, you aren't really earning a living yet - you're a student.