A hacker broke into part of the HealthCare.gov insurance enrollment website in JulyMarketWatch
And they are just now telling us about it.
An HHS official said the attack appears to mark the first successful intrusion into the website,The operative word here is "appears".
In other words, they are not really sure.
Or, they know, and just not telling.
Most companies don't know if they are hacked until someone, usually a client, notices something suspicious. If a company says they have never been hacked they are either lying or they don't know they were hacked.