The early rumours pegged the 8600K at just 3.4GHz Boost. It wasn't a case of lying, it was an opinion based on what was expected at the time of writing.
At 4.1GHz all-core though it's six threads may still be slower than an i3 with all eight threads running at 4GHz in multi-threaded tasks.
The person who wrote this article doesn't even get their facts straight. Clock speeds on the 8600K are 3.6GHz base, 4.3GHz single core, 4.2GHz 2-4 core, 4.1GHz all cores. NOT the 3.4GHz max boost clock that the author claims in the article.
Even the non K sku i5 has a higher clock speed than what the author tries to claim, so it's not like they can pretend they were referring to the non K i5. That chip, the i5 8400, has a boost clock of 4.0GHz single core and 3.8GHz on all cores.
But with the larger Cache sizes and the more physical cores instead of logical cores, that i5 8400 should still out perform the 8300 in more scenarios. Or at the very least trade blows. But the two chips will be priced very close to each other. With the 8600K supposedly being priced at the same $249 as the 7600K, the 8400 should be around $199.