To me, this sentence actually lent more credibility to the theory. Since when have characters ever done what other characters think they've been doing (e.g. how many people think Demandred is REALLY with the Borderlanders)? That's not to say that Graendal couldn't be right here, but we can't take it as fact - especially since we don't know how she came to that conclusion. Were her agents keeping an eye on him at that fortress and see him do something every ten minutes, or did she figure he's too busy preparing for war because tons of stuff is happening up there and she hasn't heard anything pointing to Moridin being active anywhere else?

The fact that she said the preparations "left him very little time for the south" could very easily be WoT-foreshadowing that that's EXACTLY what Moridin's doing - something in the South.

There is an alternative explanation, though - that Moridin's hiding because of his connection to Rand (many fans think he didn't show up at the Cleansing because of this). It's not Taimandred, but it still proves my basic point right that Graendal's sentence means Moridin's up to something else... or not up to even that, as the case may be. Either way, we can't take Graendal's conclusions as absolute truth, knowing what we do.

...Actually, now that I think about it, the latter option is more likely. But I don't see this ruling out Taimidin, either.