Its the game up until Xen, so it is more than half.
It's
way more than half. See how xen is chapters 15-18 and Inbound to Lambda Core is 1-15?
http://half-life.wikia.com/wiki/Half-Life_storyline
Due to the nature of xen's totally different environment, it would take up far more than 2/9ths of the development work. During all the previous chapters assets are reusable throughout, so just by fully populating one level, you're partially populating another. For xen though everything has to be made from scratch for xen.
Then on top of that, it's a much harder environment to get right. Because it's such a weird place and what Valve gave us originally was so poor it's open for massive interpretation.
I also expect that xen is the area that has undergone most gameplay changes, I remember having to skip parts of xen because I couldn't do the stupid jumps (that or I'd missed an interaction that opened a new route).
I suspect that they didn't start xen for quite a while after starting, it could well be one of those bits no one wants to touch until someone else has made a start on it, and since I'd estimate xen to be roughly 40% of the level design and asset creation work, I am not surprised that it isn't finished.
I also have faith that it will be finished. If the rest of the game is as good as the gaming media make it out to be, the flood of applications they'll receive after it goes live will be huge. So between their current team and any new team members they bring in, xen will get finished.
Every time I think I should offer my services to Black Mesa they announce that they're almost done, then go back on it so I've never actually offered.