OK, after further dialogue with some very bright folks on the Doom9 forum, I think I've reached a meaningful conclusion on this.
Based on the pattern of results obtained when the Checker-444 clip is 'pass-through' exported to v210 in Resolve 15.1.2, the observed 444 > 422 sub-sampling pattern is consistent with 'Bilinear Resampling', wherein each pixel is convolved with kernel 0.25, 0.5, 0.25. That is to say, the UV pair is derived by weighted area summation, as represented by the top diagram in this illustration (courtesy of the VirtualDub2 developer):
This re-sampling modality accounts for the separation of the R/B and G channel plots that is seen when the v210 exports are re-imported into Resolve. Premiere Pro sub-samples 444 to 422 in exactly the same manner, so we're in good company
Prior to Resolve 15.1.1 (up to 15.1) the 444 > 422 sub-sampling behavior was different as illustrated by the lower diagram in the picture - there it uses the 2x2 environment of a pixel and then takes the average of these neighbor pixels to interpolate the new value, as opposed to convolved area summation. Vegas Pro and Natron (which was used to create the 'Checker' clips) show exactly the same behavior. Fusion 9 also, were it not for the 'red channel' bug I posted about above.
In the context of of image scaling this mode of sampling is referred to as 'Bilinear Interpolation' (as opposed to 'Re-sampling'). Hence the potential confusion that arises when applying the term 'Bilinear' loosely in the context of 444 > 422 chroma sub-sampling. So what to call it then ? Well, the re-sampling modality that perfectly replicates this behavior in VapourSynth and AVISynth is referred to as 'Box Kernel':
Technically, both outcomes can be derived from a 'box filter' but with different chroma location, but that's what I've decided to call it to differentiate the two behaviors.
So what benefits does this new and 'improved' mode of 422 chroma sub-sampling bring ? Well that's something I intend to examine more closely (for my own interest) with 'real world' sources. The most striking benefit though is that this 'bilinear' mode of sampling makes for more consistent (reconciled) 422 sampling outcomes on export from 444 and 422 inputs - at least that was the observation in these 'pass-through' tests. That's because the 422 > 444 up-sampling is Bilinear, as it was also in 15.1. So the 15.1 'pass-through' v210 exports showed a quite different pattern of results when the 'Checker-422' clip was used for input. Again, that outcome can be closely replicated with AVISynth (and Vapoursynth) Bilinear and Box kernel re-sampling filters:
In essence, it was that behavior that sparked concern about the 422 chroma sub-sampling being sub-optimal in the first place. I'm now inclined to think it was more that Resolve was better tuned for RGB/444 input. So in that sense, I think it is an improvement.
Others, more knowledgeable than I, might weigh in on the finer pros and cons.
And with that - "A whop bop-a-lu a whop bam boom"