Yes, they came out at the same time. If you closely read the details of that event's original announcement here, you will see that Musashi was always intended to later appear in Heroics. On the other hand, there is no mention of Emperor ever being available again. In fact, he was the first character ever for which there was no information about future availability (implying no future opportunity). The Killer was the second such character.
I asked for clarification here but don't believe I received a reply.





