The "average 3 year" career is a bit like medieval life expectancy, it's pulled down by people dropping out early.
For players on the roster opening day, the average career is 6 years.
For players with 3 years of experience, the average career is 7 years.
First round picks have an average career of 9.3 years.
And if you make a Pro Bowl, your career average is 11.7 years.
League minimum is $840K. Cut it in half for taxes and agent fees and that's still $420K.
You're still talking about exclusive clubs. The PERCENTAGE of NFL players that meet those criteria is relatively small. The majority aren't dropping out early, they're replaced as soon as possible.
Their point is that the length of career is due to injury rather than ability.
And making an opening day roster already puts you 2x longer than the overall average.