It is a somewhat arbitrary metric although I would say there are two valid reasons to look at 1080p gaming (note they used 'High' settings - which in modern games really means middle options to reduce gpu load):
1: To give a valid CPU comparison you need to avoid the GPU being maxed out, running a 2080ti at 1080p high certainly shifts the focus onto the CPU. If they tested at 1440p Ultra, a more realistic scenario for these machines then you wouldn't see much difference (HW Unboxed are using a 3950X test rig, partly in preparation for the 5000 series and tested it vs their 10900K rig and at 1440p ultra there is only a 5% difference - which would inviably also be the case with the 5000 series as well).
2: High refresh rate 1080p is a thing - the fastest panels are all 1080p (240hz and even 300hz options are available at 1080p, but not typically above that). That is probably the one area where the kind of cpu performance differences AMD are talking about would actually matter. If you are gaming at 1440p or 4k ultra then even a Ryzen 7 2700X will be enough to feed most GPU's to the point it makes little difference.