If you want to go down that route then Saurfang being armed would have just dissuaded them from doing anything. He’d have to wait around until they tried something sneakier and/or until he left so they’d attack Thrall alone (who had no idea regardless).
Do you think they’d rush head first at them if either (or both) were rocking full armour and/or weapons, or if they suspected a tougher fight? They would KNOW Saurfang’s prowess/reputation, and Saurfang being unarmoured and less dangerous looking is just the carrot on the stick he’d need to get them to act first, more so if he legitimately followed them, which would make sense as he easily detected them as soon as they did attempt to make a move. Something you would do if you KNEW they were there and you were playing dumb.
Something won’t add up no matter what way you aim your magnifying glass, so you have to take it as it is presented. To me it makes sense. Saurfang knew he would be watched the moment the Dark Rangers failed to capture him initially. He’d also be extremely likely to try and keep tabs on the latest Horde/Sylvanas shenanigans.
The question is less about Saurfang and more about Sylvanas motives. As said, you have to take it at face value for now, but you’d also be foolish to assume that is the whole truth (ie. there is obviously more to it than Sylvanas taking a random notion to ‘kill’ [assumption] Thrall and/or Saurfang).
They keep saying it’s not Garrosh 2.0, so one would have to assume that this is all part of Sylvanas plan to prepare the Horde (or create a new version of it) in a different way, even if it’d be a far stretch to believe it.
Regardless, Thrall has been motivated – probably all part of the plan. It’ll all turn out to be a McGuffin, with Sylvanas steepling her fingers at the end saying “Well, I needed to get you off your backside one way or another” and “Yeah, well, I never meant to actually KILL you”.
The common speculation now is a third faction, but I believe it is just the foundation of creating a new Horde. The liklihood of three (playable) factions is minor, but there’s every possibility she (new Horde) or Thrall/Saurfang (old Horde) will rebuild the Horde one way or another.