The 6700k was superseded by the i7-7700k, pretty much the same chip, but higher core clocks.
i7-6700k + Z170 4c/8t 4.6-4.8Ghz (overclocked)
i7-7700k + Z270 4c/8t 4.8-5.1Ghz (overclocked)
i7-8700k + Z370 6c/12t 4.7Ghz (overclocked, with good cooling, it goes higher, but those extra cores make it a little harder to cool. Also more expensive then the 7700k.
What he is saying, somewhat incorrectly, that unless you already have a Z170, H170, H110, or B150 board (or Z270, B250) that you can easily get a Z370 board and an 8th gen CPU of your choice. (Availability is low though)
If you are upgrading from anything less than that you can also look at an r5-1600 or R7-1700 which would be cheaper, but not quite the same high clock speeds, but...