I fail to see how a client is a fork.
The new client is not literally a fork, but it is a code fork, let me explain.
A fork is a deviation from trunk. Something other than the official, developed, main code. Therefore if someone else develops an alternative piece of code and distributes it, it is by definition a fork.
Often the developers of the code will themselves fork a piece of the truck code to develop it separately in order to one day merge it back in. If someone develops code based on someone else's distributed code it is a fork.
Think of it like a real fork with one tine called trunk (main code). If you take the code and develop it separately you fork it because if the trunk code changes, you need to take the effort to merge it into your code, or ignore it. The two become different as soon as someone changes it. Sometimes forks change so much that they cannot be merged any longer because of some grass roots changes.
Now that we understand what a fork is, the Author of this client took code that was the 1.2 client, and forked it to create a new client. I seriously doubt that he created a completely new client from scratch.
HUD's are not forkable (unless you modify someone else's work) because there is no underlying code changes. HUD's are add-on pieces of bling that was designed for and made allowable by the code of the client. You can make a fork of a client that makes it impossible to load custom HUD's, but where's the fun in that?
Gameplay-changing mods are indeed forks, because they change the underlying code of the game and how it operates. IE, The gameplay-changing mod known as X is a fork because there is underlying code changes.
