Good tangential thinking, Tom, but I was not referring to the Bayer CFA pattern.
Indeed, I've never seen it referred to as "4:2:2" either, do you have a link for that?
As you may know, there are two parts to creating a JPEG.
One is compression. The other is degrees of color sub-sampling, including 'none'.
When sub-sampling is used the converter transforms the color space from RGB to Y'CbCr which is a two-axis space. Then Cb and Cr are downsized a) if 4:2:0 was selected but b) only one of them if 4:2:2 was selected and c) neither of them if 4:4:4 was selected.
Point being that the downsizing (called chroma sub-sampling) loses color information which is
not regained when the JPEG is decoded back to RGB, e.g. to show on a screen or to get printed.
https://en.wikipedia.org/wiki/Chroma_subsampling
Clear as mud, eh?