I don't think it's Mendicant Bias. The only connection I see is the desert setting. But from any logical standpoint, why would the Forerunners leave a giant robot lying around for an AI which they wanted to make sure never escaped its tomb/prison?

More likely, it's a War Sphinx controlled by an as-of-yet-unknown entity; going by the theories that it might actually not be immediately hostile, I wouldn't rule out characters like the IsoDidact. Though judging by 343i's convenient good-evil color coding you'd think they would've used blue or white rather than orange/red to indicate that.

Of course, it could also be the original Didact - it's almost painfully obvious that he survived, but the fake ambiguity gives 343i a chance to play the big villain mystery angle as they did with Halo 4. Only it never really worked because everyone knew who it was going to be. Then again, with the rather negative reception of the Didact's character, they might end up retiring him despite what their original plans might've been. However, it's more probable that they'll give the ol' D another chance, especially now that Silentium has clarified some of the more controversial aspects of his Halo 4 incarnation and the fact they probably have far-reaching plans for him as the next big bad.

Going back to Mendicant Bias, I think Silentium gave a pretty strong indication that Mendicant's tomb is none other than Halo 3's Epitaph. It all fits: a sarcophagus like structure in the Ark's desert sand, a hologram that might very well represent an AI, and obviously the name. The only thing not described in Silentium is the huge spire on top of the tomb, but one can imagine that was built afterwards.