Why did this A320 stall protection demo not hold Valpha max?
You need to look earlier in the video. At 3:59 the sidestick goes full back and full left. The aircraft exceeds the 33deg bank limit on automatic maintenance of pitch and bank, requiring the pilot to hold pitch and bank. 



The pilot is in command from 4:54-5:13, not choosing Valpha max. He hits the bank angle and stall limits to demonstrate flight envelope protection, the point of the video.