Sun Tzu's strategies are not all about hiding. He stresses flexibility and focuses on hiding one's capabilities when you are weak so that your enemy is unsure whether they should attack and how much damage you can inflict. On the other hand, boast your capabilities when you are strong so that your enemy's confidence is shaken and you can achieve your objectives without even fighting.
China was doing a lot of hiding in the 80's, 90's and 2000's when they were still weak. They wanted opponents to be confused about their true capabilities, thus discouraged from attacking China. Now China is not so weak any more. So their strategies have been shifted to boasting their capabilities so that their opponents would not underestimate them and would not make military decisions against them lightly. This is what the US has been doing. This is also why the US is not shy about showing off their new toys. The shock and awe would be enough to force enemy to back down. And China is moving into similar situation now.
This is especially the case when talking about DF-21, a weapon that supposedly posts huge threat to China's potential opponents. Hiding its capabilities would not do any good at all. Like nuclear weapons, actually using such weapon would be the last resort (about to start WWIII). The main purpose is deterrence. To become a deterrence, you want to be as invincible as you can. Thus, it makes no sense to hide it.