I just say it makes no sense..
At least in my opinion.

BS obviously means that the other plotlines, which are behind Rand's and Egwene's, will be brought to the same point in ToM, namely Perrin, Mat, Elayne, likely Aviendha, Lan, Loial, Fain, etc.

However, Graendal was tied to Rand's plotline, which isn't behind in the timeline, but was pretty much concluded in TGS for now. As far as we know, Graendal has no relevance in the plotlines of the others, because she had to concentrate on Rand to fulfill Moridin's orders (Graendal even says in TGS that she doesn't care about the other two ta'veren). Since ToM will - according to BS - anyway be a less focused book than TGS (probably because there are more plotlines), it makes no sense for BS to confuse the readers even more about the timeline by giving a likely redundant PoV from Graendal before her death (and AFAIK, something like this was never done in the series). As BS indicated, other characters will just mention Graendal in ToM.