Sorry to specualate, but let's assume that the "rebels" actually shot down the plane, and nothing more to the story (no hiding fighters, etc).
These "rebels" are a very diverse gang. There are surely some gang-ho drunk hereos among them, but likewise there are some very hardcore battle hardened veterans from Afganistan, Chechnya, etc. Most importantly a significant amount of Ukranian forces defected or switched sides during the early stages of the conflict.
In short, there is no shortage among the "rebels" of people highly trained in these missile systems.