Idk, you would have to ask him/her. Most people probably equate breathable air to having an atmosphere. Or maybe he/she heard that Mars air was thinner and thought it had relatively no atmosphere, which is true. Mars has less than 1% of an atmosphere compared to Earth. Edit: that's actually atmospheric pressure. Not always a direct correlation. Mars is 95% CO2 vs Earths 70% N2 which is heavier. So I should correct myself and say Mars atmospheric pressure is less than 1% of Earths. Edit2: Wow, my bad. CO2 is heavier than N2. So Mars really does have a serious lack of atmosphere compared to Earth.