I think he means that Sithis is http://dictionary.reference.com/browse/entropy (refer to definition b ). Although I don't think that's it, I can understand how one would interperet it in that fashion.
If you don't mind a bit of a tangent, number 3 is actually a better description.
It's known the Aurbis is made out of two fundamental elements. Anu and Padomay. Sithis is a tendency for the Aurbis to attain a state of maximum homogeneity.The Aurbis knows knows two periods in which it was homogeneous, at the start and with the creation of Mundus which attempted to recreate the initial state.
It's opposite, Auriel is a tendency for the Aurbis to attain a state of maximum segregation. The Aurbis knows one such period, between the realisation of Auri-El and the creation of Mundus. In this period the et'Ada started to last and seperate themselves into Aetherius and Oblivion.
These conclusions should at the very least be considered odd because Anuiel and Sithis are considered to be interchangeable with Anu and Padomay or in other cases, their respective souls. Yet here, Anuiel and Sithis act tangentially to Anu and Padomay as at any time something is made more Anu-like by Anu-iel, something else is made more Padomay-like or when something is made less Anu-like by Sithis, something else is made less Padomay-like.
To put this in the terms of the Loveletter. Sithis is the forces that joins, that is love. Auriel is hate. Yet Sithis is also death, and Auriel is life. To be immortal like the et'Ada is to hate. Only love can die.
To love is to die. Mara has blood on her hands.
To die is to love! Mephala goddess of Love!
Shezarr! Dreaded father of murder! Lover of Mara!
Mephala I take thine mourning veil from thy face and show thee to the world as Mara!