Wow, that is absolutely amazing, obvious, and yet I hadn't considered it before! Offhand, I'd say you're right. Marcel was his name, right? I wonder how he would have factored into the plan by being able to use his Titan-shifter abilities. And it also seems to confirm that humans who are transformed into Titans have to eat other Titan-Shifters to be able to Titan-Shift themselves back into humans again? If that's the case, then Eren just got lucky, since if the digestion were completed, there would be a Santa Claus Titan shifter instead. It also begs the question as to why Marcel couldn't transform inside Ymir like Eren did, but then again we already know Eren is special even among Titan-shifters!