Interesting article, I believe based on the methods section this is a computer simulation.

The t-cells from flu infections clearly didn’t provide complete protection against Covid-19 as 600,000 Americans died. But perhaps the death toll would be even worse. We’ve all seen the news stories where multiple family members in a single family die of Covid-19 and the explanation for how that happens with a virus where only approximately 0.6% of the infected die has always seemed a bit unknown (especially when it’s e.g. two people in their 50s both dying, who probably each had a survival chance of 99.9% or so based on their age).

I’m sure the authors would agree that even if you had a clinically diagnosed flu infection in 2019-2020 it’s probably a good idea to get the covid-19 vaccine anyway.