It's irrelevant to make a comparison between an unclockable and a clockable CPU, if it would have compared that Ryzen 5 against the Intel 8400k that would be a different story, in which the Intel CPU would have been litteraly crushed the Ryzen 5.
They were compared because they were almost identical in price:
https://pcpartpicker.com/products/cpu/#s=60,12&sort=price&k=33,30
-- $175 USD for the R5 1600 (comes with its own cooler, which allows for light to moderate overclocking
if you want, but the main benefit is you don't have to pay for another cooler)
-- $179 USD for the i5-8400 (comes with its own cooler)
-- $238 USD for the i5-8600K (does
not come with its own cooler, which means you have to plan to pay another $20-50 USD
even if you're going to stick it into a B360 chipset motherboard)
Note, that's an extra $80-110 USD you have to spend for the i5-8600K (including cooler)
even if you go with a non-Z370 motherboard. Note also that, when they originally tested the i5-8600K (& the i7-8700K) against Ryzen (https://www.techspot.com/review/1505-intel-core-8th-gen-vs-amd-ryzen/), the 720p numbers may have shown a decent-sized gap between Coffee Lake & Ryzen in performance...but not only would the gap only be noticeable on monitors with refresh rates
over 144hz, but the gaps were drastically reduced (almost to the point of non-existance) as you increased the resolution (especially once you hit 1440p). Nor was the 8600K noticeable faster than the 8400; despite its much faster stock base speed (3.6 vs. 2.8GHz), their max Turbo speeds are nearly identical (4.1 vs 3.8 GHz with 5 or 6 cores, 4.2 vs. 3.9 GHz with 2-4 cores, & 4.3 vs. 4.0GHz on a single core). Even the i7-8700K's improved performance was due more to clock speed than to having double the threads.
Now, maybe the extra $100 doesn't bother someone that's having to do a
complete rebuild of their system: storage (I.e. because they're upgrading from a 7200RPM, or even a 5400RPM, HDD), OS (I.e. they were still using Windows XP or Vista), removable drive (I.e. old system only had a CD-ROM drive), new case, new GPU (I.e. old system was "rocking" a Radeon HD 4670). etc. On that kind of build, with the current GPU prices, an extra $100 isn't going to break the bank.
But... if you're reusing your existing case/PSU/SSD, you're holding off on a "new" GPU because your current one still works (especially if you were lucky enough to pick up a GTX 1060/1070/1080 or RX 480/Vega before prices went up), & you're only looking at CPU/motherboard/RAM replacement, then $100 is a much more significant part of your expense.