Well Vader and Palpatine survived the force blast at the end. Maybe Marek did as well and Vader just assumed he was dead. (But then if he was alive they would since his life force.)
Of course Vader could have used the force to conceal Marek's life so he could heal him again.
Though in the SW EU it is not unheard of for one with the force to cheat death. A few sith (Darth Anddedu) were able to sustain there life beyond death. It's possible that Marek learned something similar, perhaps as a force ghost.
One thing I would have to say is that I don't think the time period will be set before or during the time of TFU, but after it. Because as far as we know since Vader took him in as a young boy, he had been training under Vader as a Sith apprentice. Why would he, during that time, be at an arena fighting with twin blu sabers? Also to point out that in the trailer it shows him using an incredible amount of force lightning, he was not that powerful until closer to the end of TFU.