There method seems fine to me, here is what they mention on there site:
The 8th and 9th-gen Intel Core processors were benchmarked on the Gigabyte Z390 Aorus Ultra, using the same DDR4-3200 CL14 memory, but they were cooled using the Corsair Hydro H115i RGB Platinum 280mm liquid cooler. Do note the Intel CPUs are not TDP restricted as that’s not the out of the box experience, so we are showing the absolute best case scenario for out of the box performance. Finally, our graphics card of choice was the MSI Trio GeForce RTX 2080 Ti.
Next, Intel has a turbo table on there CPU's and will boost accordingly. Hardly AMD's fault that there method gets closer to the limit out the box then Intel's turbo table does. In terms of cache, not many people bother OCing that and it rarely changes the number. At least with my 8700k overclocking the cache does nothing.
In regards to manually overclocking, hardly make things equal, more so the 5.0 Ghz you say. A famous site which sells chips based on clock speeds they hit and are stable at suggests only 35% of 9900k's were fully stable at 5.0 GHz and 87% at 4.9 GHz in stress tests, which is a reasonable ask as the Ryzen chips are similarly stable in stress tests out the box (I would hope). So then its a matter of, what is a suitable number to OC to that ensures everyone happy.
Now do not mistake me, I agree, there is a little bit more overhead on the Intel side in terms of overclocking it seems, but for the purpose of an apple's to apples comparison, out the box and a appropriate set of RAM seems to be the most easy way to compare these things.