User blog comment:Jopjopjop/Volume 20 Spoilers/@comment-178.128.252.204-20150826194658/@comment-26209278-20150831165532

........ Pretty sure EX IS canon to the LN......

And Issei can't just summon the Diabolos Dragon armour at will, it takes a lot of effort..... if he actually can do it again, we don't know.