We just don't every really know, sometimes the authors of 40k novels really do know what is coming out in the next codex, and include some really cool descriptions of what we might be seeing. This could be the case here. With Path of the Seer there is a flashback of a battle against the Necrons and this rumor is about that.
Please remember that these are rumors, please take with salt.
In the BL book PAth of the Seer, out this weekend theres a flashback memory of a battle against the Necrons
Gold coloured warriors, Flayed ones, Destroyers and Scarabs are mentioned, but not by name
Also there are smaller monoliths that act only as gun turrets, and a quote;
"One of the suns was setting and in the dimming light the pyramid changed. Another section of the gleaming metal slid away to reveal an immense hanger like space. From the darkness emerged a terrifying apparition, glowing with green energy. it looked like a cross between a building and a warrior, a huge construct with a dozen heads and batteries of weapons set about a complex, ever-shifting geometric core.
The necrontyr war machine loomed over the battlefield, sheathed in a baleful glow that warded away the blasts of bright lances and scatter lasers. An orb at its centre spun faster and faster, crackling with energy that crawled along arcane circuitary to the blisters of the weapons turrets.
With a blinding flash, green lightning arced down upon the eldar army, shredding tanks and aspect warriors in a barrage of pyrotecnic destruction. Whole squads were vaporised. Falcons exploded or were sheared into small pieces"
I can't think of anything matching the description in the current Nec book. Now I know BL books aren't the best place to go for rumours....but could be something?
It also mentioned Scarabs destroying Jetbikes and Tanks, by climbing onto them and self destructing in large numbers