I would say that no episodes since he started medications 6 years ago only make it an issue for people who don't understand the problem.
Most understandings are simplistic at best. Medication only manage the problem, they are not a cure. There are a myriad of things that could go wrong at any time, including building up an intolerance to the medications, or side effects that could prevent you from continuing the medication, or the sense that you feel so "well" you don't need medication any longer and stop taking it.
I've been through this with a family member for over 30 years - battling depression and anxiety disorders, etc. is like holding a beach ball under water; you have to constantly stay on top of it, otherwise, it pops out from under you and you sink right to the bottom.
It's to Greinke's credit and his doctors, that he's been well for this long and there's nothing to suggest that he's at risk of a relapse, but the underlying cause still exists and could still be a concern.