I’m sorry that’s a misunderstanding.
I reported that shell menus are hard to see against dark backgrounds. I said that one solution to this problem would be for shell menus to use the same colors as dark-theme app menus.
I did not wonder why “menus have different colors … in light and dark versions of the gtk theme”. They have different colors for the same reason that most other controls do — because that’s the whole point of having light and dark versions.
That could improve consistency in one way, by reducing the number of colors used for controls overall.
But it would make consistency worse in a different way, by making the light theme less consistent with itself.
And it would do nothing to help the original problem of shell menus being hard to see against dark backgrounds.