I am answering a person seeking advice on booking with Air China. Recently I was considering booking with them myself, and only later saw such a notice on their website. Had I booked beforehand, I could have potentially ended up unable to receive the ticket I paid for.

I did not make a booking with them in the end, so I can't say how difficult it is to recover from such a situation. Maybe it does work with @gmail.com addresses as you say but if it doesn't, the airline certainly shrugs off any responsibility for this.

If I were about to book with Air China now and were asking for advice, I wish someone told me just that, so that I could make an informed decision, which is primarily why I posted in this thread.

If it's such a "non-issue" to you, no one is forcing you to respond, feel free to move along.