Here's the actual line from the SI article:
"By the time they have been retired for two years, 78% of former NFL players have gone bankrupt or are under financial stress because of joblessness or divorce."
That "or" certainly broadens the claim well beyond "78% are bankrupt." The operative word is "joblessness," since that situation would create distress for anyone--ex NFL player or ex-WalMart clerk--forced to look for a new career.
http://www.ehow.com/facts_5005211_what-average-salary-nfl-players.html
"The average NFL salary is a shade under $1.8 million in 2009, but that can be a misleading number. Less than a quarter of the roughly 1,800 NFL players make that amount, and less than half make a million dollars per season. In reality, the NFL isn't nearly the "easy money" venue that many fans think. The majority of players never even see a million dollars for their career."
Median NFL salaries 2009-10