That absolutely should have worked, or otherwise Thanatos should have been able to resist this, right? But possession doesn't really happen to gods, so he was unprepared for the possibility of such a thing. So he is disoriented, with something else driving his body, slamming his hand into the wall, making his jaw clench and turning him around to face Orpheus.

It's unpracticed with using his body, thankfully for Orpheus, or he likely would have just teleported to Orpheus, or just used his scythe in it's pursuit, but as it is, he stumbles a little as a leg is forced forward, then the other, in a slow and jerky march. It gives Orpheus a few more moments to decide on what he wants to do before Thanatos' body is fully under the control of the invader, and the jerkiness in his movements drops as he dashes for Orpheus, swiping at him with his right hand; the one with his sharp-fingered gauntlet on it.