It seems a lot of people think that all that was released was the uncompiled source code to HL2. This is not the case.
The guy who broke into their system basically downloaded a snapshot of their entire development tree for pretty much all their products at that time. He only released the source at first, but within a few days he had released a complete and somewhat playable version of HL2. It had several assets that were out of date for that version of the executable and would not load, and tons of maps spanning the entire length of the game in various states of development (from the basic orange/white completely unfinished maps to what appeared to be near-complete versions of other maps). Yes it included the E3-specific content, but that's not *all* it included. It was not really what you could call an "alpha" because it wasn't intended to be played by anyone at that point, in fact it'd be impossible. Generally an alpha is a feature complete or at least playable version of the game. The best name you can apply to what was leaked is a "development snapshot".
Later he also released various HL mods in various states of being converted to the HL2 engine, including a version of what is now known as CS:Source with a single map, and HL1 completely recompiled for HL2 with physics objects and everything.
He also released Ritual's version of Condition Zero, in addition to an early version of the final, TRS version of CS:CZ.
This was a HUGE content leak. HUGE. Anyone who tells you differently is either ignorant or lying.
How do I know all this? Uh... *runs away*