The paper you mentioned, they reported the kinase activity was specific for that cell type. The phosphorylation of Akt both at Ser473 and Thr308 are required for complete activation in most of the cells. For further information please see the articles below:
Phosphorylation at both side are important to show AKT its activity. Phosphorylation at ser 473 wont take place until and unless phosphorylation at ther308 is there as phosphorylation at thr side is generally called as primimg agent