Really? I didn't know that. How is that even possible? Is it just some classification trickery?
More or less.
Ok the whole thing is like this.
A plugin is code that directly interferes with the basic minecraft code. It works, but with everybody messing with the same code in different ways you come into issues when someone wants a hunk of the code to one thing and another whats it to go another thing.
Forge is a code that connects to the basic vanilla code. And lets other cods work with it.
It is designed at a basic level so that rather than others making the code do one thing or the other it works with there code so it can do both at once.
If you would like to know why that is so important the fallowing will explain.
The problem with how everything is now is that every time mincraft updates all the plugins have to change to reflect how minecraft changed. The plugins go looking for particular terms at specific locations that work in a particular way.
Now the API is supposed to be a subset of the minecraft code that the base code will connect to and ideally will never change. So plugins would never have to be updated no matter how vanilla changes. Though plugins will still be updated for simple improvements and bug fixes.
If the API gets finished even if minecraft changes versions you would not have to wait any length of time to use that new version in a pack.
But as it's meant to be something that is never changed. As in some kind of perfect code that can handle anything a plugin dev could ever wish of it, well that's more or less an impossible thing to do. And even a version with the most basic functionality will definitely be hard to do and time consuming.
And while trying to do so they finally took a good hard objective look at the basic minecraft code and realized much of it is total crap.
It wasn't crap when it was written but as MC has changed over the years becoming nothing like it was when it was originally created. And after year and year of changes by different people with different ideas and different ways to code. Well now they need to clean up all that crap before they can really hope get anywhere. And doing so requires changing a massive amount of that old code.
Of coarse changing all the things like that means all the plugins also need radical changes. Including forge. So they not only have had to figure out how MC has bin recoded but also more or less re wright the plugins from scratch. And only then once forge had bin more or less rewritten could the mod deves start to work. And they had to figure out how not only basic MC had changed but forge and not those two now interact. Only they could they work on there mods. Which also almost required a complete re wright.
And that has bin burning out mod devs left and right who only code for minecraft for fun. :/
So yah that API is kinda important. But only if it ever actually gets finished.
But at lest with the current changes making mod packs are easier. So yah there is that. Making good packs. Still a time consuming challenge.