Yeah, I was also about to point that out. The 290X was faster in SP while the GTX 780 was quite a bit faster in MP. I'd attribute that to the heavily multi threaded DX11 drivers that NV has, and AMD doesn't.
But it is funny, that in the XFX HardOCP 290X review people were called out for questioning the results which were quite simply, ridiculous - these were cards not overclocked and for some reason mysteriously outperformed reference by a mile. Yet here we are. Full circle. With some people questioning results from PCLab, a reputable website, just because they don't favor the side they prefer. Sounds pretty hypocritical to me, pot kettle black and such, but whatever. Just shows that there is no moral high ground here when it comes to benchmark results.
Again, I think the Mantle results at that website are in line with everything else we've seen. Big benefits for APUs, and the GTX 780 takes the lead in MP testing due to nvidia's DX11 multi threaded drivers. I'd point out that AMD does not have MT DX11 drivers, unless something changed very recently.