It depends on the aspect ratio though. But stating the horizontal resolution is the standard now when talking about the numbers of Ks.
You are right, for a regular TV set, the aspect ratio means 8K would give you 7,680 x 4,320 pixels. ( or there abouts )
But VR headsets are different aspect ratio than a regular TV, so 8k for a headset would be roughly half the number of pixels.
I'm not defending that way of quoting the resolution though, it is just what seems to have become standard in recent years, and I can see the benefits to it. It just evolved to be useful.
We've already had these standards over the years:
* Because CRT TV's were originally round the standard was to state the Diameter of the screen, which then becomes the diagonal size as is still used today, rather than the width or height.
* In the 80s the resolutions were often just quoted in full, like 320x240. Maybe because the numbers were smaller, so didn't take up too much space
* Then it became all those VGA, XGAs for a bit, that I found a bit confusing to remember.
* Then for a while it was just the vertical resolution that was used, so 720, 1080 etc..
* Then Megapixels were the thing, not so much for displays, but in a sense is the truest specification, just that the numbers are so big. So 4k TV would be 8.3 mega pixels
* And now we just seem to quote the horizontal resolution instead, that taken together with the aspect ration is enough to determine how many pixels you are really getting for your money.