I didn't fully read it but doesn't it look like there is potentially a big survivorship bias in the data? They touch the subject with their 3rd hypothesis but

I didn't fully read it but doesn't it look like there is potentially a big survivorship bias in the data? They touch the subject with their 3rd hypothesis but in general it's possible that you still have a higher life expectancy being born from April-June in the Northern Hemisphere because you have increased chances of reaching 50. So why this limit of 50 years? What is the expected lifespan for newborns depending on their month of birth?