How is this comparable with the giant?
Because, to me:
Potions are something that are available to all players of the game on the front end. Giants can only be accessed through either commands or NBT editing. The fact that one thing is broken with one potion does not mean that it is an idea that fell by the wayside of the developer, it just means that it's broken, and is a bug.
How is this just something that isn't supported by the developer?
I think that you misunderstand.
I can drink a potion that has saturation as a custom potion effect, and receive the saturation effect. However, I can not splash a splash potion that has saturation as a custom potion effect, and receive the saturation effect.
The fact that this is not implemented is the bug, as this is not the case with literally every other potion effect, such as Health Boost (Id:21) which is an effect that cannot be obtained through, as Torabi said "...normal gameplay mechanics."
I think that this should be reopened, unless the fact that I can splash myself with health boost is a bug.
The lighter pixels in the bottom left make me think it's the older one, along with the shading on the top row of pixels.