Apple denied the claims in a blog post:
Over the course of the past year, Bloomberg has contacted us multiple times with claims, sometimes vague and sometimes elaborate, of an alleged security incident at Apple. Each time, we have conducted rigorous internal investigations based on their inquiries and each time we have found absolutely no evidence to support any of them. We have repeatedly and consistently offered factual responses, on the record, refuting virtually every aspect of Bloomberg’s story relating to Apple.
And now Apple's Vice President of Information Security has written a letter to Congress, which SecurityWeek has posted.
Right at the top, the letter says this:
You should know that Bloomberg provided us with no evidence to substantiate their claims and our internal investigations concluded their claims were simply wrong.