It's a marketing gimmick. It seems quite clear Xiaomi is releasing very limited stocks for its online sales, hence the buzz generated from being sold out in x minutes time.
They are trying (and maybe succeeding) in creating a "scarcity" for their product, so that people will "gian" or crave for it. It's like when you see a long queue for food stall and you think the food has to be good or else why are people queuing?
Making use of social proof techniques to make their product more desirable.
I regret wasting time camping for the online sales and will not waste any more time to be party to such marketing tactics.
Xiaomi's marketing folks should be ashamed of using such tactics and wasting people's time.