As the paper itself clarifies, the worst case model for CO2 emissions still wouldn't have us hit those levels for 75 years.

> If fossil fuel use continues to rapidly expand over the remainder of the century, it is possible levels could get that high. The Representative Concentration Pathways 8.5 scenario (RCP8.5), a very high emissions scenario examined by climate scientists, has the Earth’s atmosphere reaching around 1,100ppm by the year 2100. But this would require the world to massively expand coal use and eschew any climate mitigation over the rest of this century.

So yes, it is a scary theoretical possibility. One that helps show why even minor mitigations of CO2 emissions growth are better than nothing. However, it is a risk that is not imminent and is something that even our current anemic response to global warming will probably avoid.