We do use the dividend valuation formula (it works for any inflating perpetuity). However it only gives the PV if the first flow is in 1 years time. Here, the first flow is in 5 years time (which is 4 years later than 1 years time) and therefore the answer needs discounting for a further four years to get back to the PV now. 0.636 is the discount factor for 4 years at 12%.