A few things.. the thumbnail shows an entry level kingston sata ssd, but in the video we see a WD Blue m.2 sata ssd installed which is noticably faster and competes with the 860 evo for fastest sata consumer ssd.
The gen3 ssd looks to be a cheap 2x pci-e drive with a slow controller and possibly even QLC flash, which would be a fair comparison to the cheap kingston drive shown in the thumbnail but would be slower in actual use than the WD Blue sata drive.
Gen4 ssd's currently are tuned for bandwidth and not latency, to the point where AMD told wendle not to use one when benching threadripper 3000 because it will slow it down compared to good gen3 drive (like a corsair MP510 or samsung 970 EVO)
So really the problem was with the drives used, but that wasn't discussed in the video and was possibly even misrepresented by the video's thumbnail because if people buy the sata drive shown in the thumbnail performance will be worse than every drive shown in the video, sometimes not even reaching more than sataII speeds.
veritronx got a reaction from greenmax in Faster Storage Matters - $h!t Manufacturers Say