Xiaomi is 3.5 years old, currently only sells in China and in Chinese market it has already overtaken Apple in market share.
I think comparing them to Apple on the basis of sold phones in China is justified.
Looking into my crystal ball, I further think that their strategy is so brilliant and their execution is so good that we're witnessing the rise of a new giant.
Give it 5 years and they'll be in the same league as Apple and Samsung on a world-wide basis, not just in China.