Well, he might actually retreat if he's aware that the VI isn't that far away, since that would mean he has to fight an offensive battle while being heavily outnumbered.
Though politically and strategically he might feel compelled to. Politics-wise, retreating from Daurstein without firing a shot is probably a worse look than not trying to retake it in the first place. And strategically, while we've been given orders not to cross the Raoille, von Trotha doesn't know that, and he might be looking at the III Army tearing through Gelle-Musselmond and thinking that if he doesn't blunt the V/VI Armies now, Engelsberg might be facing a two-pronged attack from both the west and south in another week, with his single army only able to respond to one of them.