She could be taking an entirely seperate action to be rid of the residues left by a weave already undone. The invisibility weave is no longer operating well before that quote comes up. When Aviendha and Elayne were unraveling their gateways, the apertures stayed visible until the last moment. Things were coming through Elayne's with no ill effect until she let it all fall apart. So rather than undoing the invisibility weave so it leaves no residue, she might have been hastening the dissipation of the residue. It seems to take a while, so it might be one of those things that would normally be more trouble than it is worth, except for their happening to be in the Blight at that moment.

Moiraine's explanation points more towards an act that is discrete from her removal of the concealment weave, IMO.

As for the idea that she is keeping this unweaving skill secret, in Lord of Chaos, there is discussion about the issue of Aes Sedai retaining secret weaves they work out themselves. That particular weave Moiraine uses, the invisibility ward, is one that is learned from Moghedian, but which no one in Salidar will admit to knowing anything about. It is possible that Moiraine might believe that her ability to eradicate all traces of that weave is something peculiar to, and a necessary part of, that invisibility weave.. What good is a weave that protects you from sight, but can be seen itself? It would be like answering a knock on the door by saying "No one's here!" So maybe her ability to unravel the weave is limited solely to that particular weave, and she only believes it possible because of the concealment-oriented nature. Since it seems to be a personal specialty, her reticence with the other sisters regarding that Talent might not have allowed her to realize the significance of what she is doing with it. Also, the blowback of a botched unraveling might be less for that particular weave.

Aviendha describes how failing with a knot of Air resulted in the sort of physical force one might expect from Air, and the gateway was the sort of unpredictable thing you might expect from messing with the fabric of reality, but Moiraine's concealment was just a manipulation of light. Maybe the worst side effect she suffered from screwing it up would have been a flash or flare, and she thought it a harmless risk for that particular weave, and one not worth taking for anything with a more tangible effect.

The whole skill of unraveling a weave seems to radical a discovery for a sister, even one as unorthodox as Moiraine, to keep for herself. If that is what she does with that weave in the Blight, I think a good case can be made that either she believes it is simply a feature in keeping with the weave's purpose of concealment or that the intangible nauture of the weave's function means it is one of the very few it is safe to unravel, she doesn't try to unweave anything else.