The writer in the article suggested that Bill Walsh created an offense that took advantage of a rule. All Bill Walsh did was create an offense that allowed players to do things they did not traditionally do. Actually they were running similar offenses with John Unitas in the 50s and 60s, Walsh just had success with the concept and created more plays with a specific concept. He did not take advantage of a rule to run that offense. And the article was a little misleading by suggesting that all players could go out for a pass. That is also not true; only 5 players can go out for a pass. There are just 11 eligible numbers but most are illegal by position. This was never stated clearly in the article.
You have to read the entire thing, not pull out one or two lines.
Peace