That is how I understand it according to the things I have read. I thought it was cult of the damned, but it appears to be the cult of the forgotten shadow. The following passages are taken from the horde players guide:

The Forgotten Shadow is a corrupted reinterpretation of the beliefs and traditions practiced by the Holy Light, and is the religion of the Forsaken. The religion of those who follow and use the shadow is called the Cult of Forgotten Shadow (aka Cult of the Forgotten Shadow).

(I don't see anything about mental magic or elves or a dark elune there... I see those that FOLLOW AND USE THE SHADOW are members of x organization.)

(As to Forsaken being able to wield the light, HPG 180:)

While the undead can no longer use the Holy Light, and have learned how to use the Shadow, they also preach that there must be a balance between Light and Shadow, and that they must learn the Light as well, but never forget they were born from the Shadow.

(A statement that seemingly contradicts itself... The undead can no longer use the holy light, but they must learn it? Which is where I got the idea Forsaken can't use the light, or can they... it's not quite clear.)