User blog comment:B214/Question and Answers 4/@comment-97.103.233.248-20150324035009/@comment-202.152.202.239-20150324130104

There are a lot of theories regarding that issue, one of them is that his Armour still contains some of demonic power (do note that his armour is not entirely his original body). Some people would prefer to say that those abilities are exclusive to Issei even without his demonic power, either way you can choose any kind of theory as you see fit.