The i7-6900K is a Broadwell-E part (i.e., not their "mainstream" high-end parts)—while it implies an eight-core part against an eight-core part is competitive,

The i7-6900K is a Broadwell-E part (i.e., not their "mainstream" high-end parts)—while it implies an eight-core part against an eight-core part is competitive, it doesn't say anything about four-core against four-core (which as far as I know at Intel are different designs from different masks, and that's ignoring the fact that now the competition there is Skylake and a different microarchitecture too).