I did NOT see the part about the 300 series board, so apologies for that, but there's no need to became an asshat over simply making sure you understood the implication. You'd be surprised how many people think that because 6th, 7th, 8th and 9th gen Intel CPUs are all the same socket, that they are compatible. So, take it easy man.
As for the K version, even though your board does not support overclocking, that does not make the K version a waste of money. For one, the K samples are binned better, so they tend to have better thermal properties than the non K versions. For another, they generally support higher base and turbo clocks, so you still gain some performance beyond what the non-k SKU would offer you. No need to go all captain crazy pants over a missed statement.