Two things you forget to mention:
1) That's not a secret, and the Russkies originally claimed that they stole the BuK from the Ukrainians in the first place (but that is common practice. They always claim to "capture" something from the Ukrainians before shipping it over, and these BuKs were captured in NATO satelight photos on both sides of the border, and identified as Russian weapons with Russian officers training Russian separatists.)
2) The BuKs are confirmed to be in "separatist" held towns, as seen by journalists from AP, geo-located photographs, and confirmed by the "separatists" themselves in their web postings (prior to the shooting).
I didn’t forget to mention anything.
The author is pointedly ignoring an obvious and legitimate possibility, it’s as simple as that.