The CVE and the patch came out on March 7. Exploits were already being detected in the wild at the time by perimeter security vendors. See http://blog.talosintelligence.com/2017/03/apache-0-day-explo...

Equifax "believes" that the hackers got in on May 13. They had some kind of intrusion detection system that finally detected the intrusion on July 29. 5 months after the "Critical" CVE alert went out. During that time security vendors were adding firewall rules to stop the attack. But apparently Equifax didn't have any other security in front of the Struts server.

That just seems like unconscionable incompetence and malpractice for such a high value target.