Ah! He's remembered!
Ahem... Ok, I went back and looked at it and remember why it made sense to me then. Morgoth thought they were all going to die, so it didn't matter that Angrod knew. He wouldn't have time to tell anyone.
And it is triggered by Angrod's hope in the Fëanoreans, a place we brought him to (a bit prematurely in my mind) to give him a full arc, because this was the last time we would see him.
If he is to survive, it would be kind of better if that change had a stronger motivation, like the actual arrival of the Fëanoreans. In which case, his attempt to enliven Edhellos' spirit is actually hollow. He doesn't actually believe it. She could mock him for that.... I don't know. Initially, the reason Morgoth did that was because he couldn't resist tormenting Angrod. And he has no problem tossing Edhellos aside.
I'm just trying to think this through, I don't actually have an endpoint in mind for this stream of consciousness.
Edit: Ultimately, I don't see Sauron doing evil just for kicks and giggles, or being as willing as Morgoth to throw away a useful tool. Sauron also doesn't seem like the kind of person (at this stage) to assume this battle is a foregone conclusion. And that is all assuming he even has full knowledge of what Morgoth is doing with Edhellos in real time.
At any rate, I would say that if Sauron were present at the battle or involved in some way, it really damages the believeability of the fact that Maedhros gets the drop on them immediately after. The cavalry have to go all the way around Gothmog's army without being detected by anyone they can't immediately run down. I brought this objection up before. I just don't buy Sauron on this field.