It is a valid point, but the problem is how would the rebels know how to operate the equipment? Most Russian/Soviet gears are dated and hard to operate, they require special training. Those are probably produced near the end or just after the Soviet era, definitely no fancy easy to use GUI like those in NATO equipment. The national level chaos didn't start until March/April and real large scale military conflict didn't start until two months ago.
It's impossible for a bunch of random people to just capture bunch of sophisticated SAM units and be able to operate them without professional assistance, especially downing a high flying jet cruising at Mach 0.9 at 10,000m altitude.
It's true the Eastern blocs have extremely high ratio of university educated population, but those are not toys from Walmart or Toys-R-Us, they are sophisticated weapons. Even just learning the acronyms and the logics would take weeks, not to mention fully using them.
That's why I believe it's an active SAM unit of Russian or Ukrainian military that did it. This conclusion is not politically based, but simple deductive reasoning.