I'd often wondered about this myself, since I can find no reference to it rules-wise. I think that your suggestion, AndrewChristlieb, is the likely answer. I had also hypothesised that it was a legacy of an older Necron list (and was some kind of area affecting weapon, like the star pulse is now). Anyway, there are multiple references to it elsewhere (but unfortunately, none of them come to me off the top of my head!)... so perhaps it is to help with the 'mysterious nature' of Necrons.... There are also references in the background about some Scythe-classes having Sepulchre-like structures, but I've never seen an official rule to accommodate this.
On a more solid note, if it doesn't refer to the Sepulchre attack, what do you think it would do?