Not exactly. There is a distinct difference between a soldier and a combatant. A soldier is trained and is a member of a standing military. The separatists can at best be described as "irregulars", or insurgents or rebels if you want to go with slightly more charged terminology.
Yes, really? With that definition it'd be impossible for a new military to ever be created, because anyone who joins and fights with one is not joining a standing army therefore cannot be soldiers. That is obviously nonsense, it must be possible for someone to be a soldier in a newly formed army, which is what it looks like is happening here.
Additionally, you claim that the fighters in Donetsk cannot be soldiers because soldiers are trained, and then immediately claim they're receiving training from Russia. So which is it?
And given the fact that the missiles were launched from inside territory controlled by the rebelsis a very important detail. Why would the Ukrainians have anti-air equipment deployed in an area they do not control, against an enemy with no air power?
You're quite right - it probably was the separatists. This does not change the accuracy of the Wikipedia edit that's being discussed, because unless/until the separatists win, they are still Ukrainians.
Although I'd note that given the amount of bullshit emanating from all sides in this conflict it's hard to really know anything about what's going on. The area of Ukraine that's in revolt is next to the Russian border, which is exactly where you'd expect the Ukrainian military to have had lots of soldiers and equipment stationed. Missiles might have been trucked over the Russian border, or they might simply have been there already. The separatists might be being trained by Russians (this would be unsurprising and not exactly unprecedented - see how the USA supported rebels in Syria), or alternatively they might be operating the equipment without really knowing what they're doing - indeed, having no clue what you're targeting would be rather indicative of not being properly trained, no? Or perhaps they're being trained by people who are ethnically Russian but lived in Ukraine at the time of the rebellion, or one of many other more complex cases that won't neatly fit into the "Putin fired the missiles himself" story the west is busy pushing.
All we can say for sure is that whatever you read about this incident is going to be full-blown propaganda, and should be treated as such.