The call in system is also garbage.
In October, I flew SFO-PEK on their 747-8i, and also couldn't book a seat.
I called THREE times, and waited on hold for over an hour each time, and they STILL couldn't handle it. I was actually told a different story each time I called. First call said "no seat available; this flight all full". Second call said "I've selected seat 159Z for you." Third call said "We do not assign seats in economy class over the phone."
They did actually reserve me a seat at one point- a window in one of the very last rows (but not so far back as to be in the coveted 2-seat sets).
At check in, I was able to move to a window seat further forward, and actually ended up in a fantastic seat. Last row of zone 4, so seat 47A which had FULL recline in spite of what seatguru tells you.
And I also discovered that, once in the air, Air China is really a pretty great ride. Seat pitch was good, cushion thickness was great, PTV was good, food was good, etc.

But no, I invested way too much time trying to get a seat assignment. The operators for the USA number are clearly selected based on their ability to speak Mandarin.... NOT English. Communicating with them was challenging. YMMV.
So, best of luck to you on that!