Of course a purpose-built PC will outperform an iMac, dollar for dollar. That's not exactly news. The iMac also has a severe price-to-performance disadvantage here by including an expensive 5K panel, while the PC is tested with a much cheaper 2560x1440 monitor.
However, the disappointing thing to me here is that you can't reach performance parity with the iMac by throwing more money at it. The iMac tested here is a completely maxed out machine.
The Skylake processor in the iMac is something the end user cannot overclock (without doing something extreme). And none of the Mac Pro cores are as fast. And Apple doesn't water cool their machines. They simply don't offer robust overclocking options to us. So, for a single-core, the 4.0 GHz Skylake in the iMac is the fastest thing Apple offers.
If there is an underlying, valid core criticism in this article, it's the same one everybody already knows about: Apple doesn't offer their end users a truly competitive array of system configurations. Instead, they force their users into carefully defined and limited product tiers, each one costing significantly more than the last.
However, the disappointing thing to me here is that you can't reach performance parity with the iMac by throwing more money at it. The iMac tested here is a completely maxed out machine.