It is. Your subjob will not affect you primary job lvl, it will only half it if it is selected as your sub.

Example: lvl 18 whm gets subjob quest done, all subjobs are at lvl 1

lvl blm to 10

BLM/WHM 1/1 2/1 3/1 4/2 but whm will always remain at lvl 18

WHM/BLM 18/9 20/10 22/10 BLM will remain at 10 unless further leveled.