The author didn't cite any sources but I suspect they were alluding to the fact that we know (roughly) when the field reversals happened in the recent past, and we also know when the mass extinctions were, and one can then see that the field reversals don't tend to cause mass extinctions.
I guess the main reason is that the Earth's atmosphere also provides a lot of radiation protection, regardless of what the magnetic field is doing.
I guess the main reason is that the Earth's atmosphere also provides a lot of radiation protection, regardless of what the magnetic field is doing.