The "source" is not named in the entire article. He is simply described as "source familiar with the situation". The opinions that follow are his. He also notes, correctly, that if this legitimacy business goes too far, then the legitimacy of the USSR and other states is questionable as well.
But the issue here is not the quality of the General Prosecutor's decision, but the fact that the review is taking place, alongside with the decision that the transfer of Crimea in 1954 was illegitimate, and we know what happened to Crimea.
I wasn’t surprised about Crimea, there was no way Russia was gonna give up its only warm water port and large naval base there, especially with the Crimean population being 75% Russian.
As far as the rebel held areas to the east, I doubt at this point Putin will settle for anything less than some sort of at least limited autonomy. He knows Western Europe doesn’t have the stomach for bringing NATO into a serious confrontation there, which is why I believe Obama has been holding off despite pressure from Congress to ramp this thing up. There will be some transfer of weapons, but probably not enough for Ukraine to achieve a decisive victory, leaving a smoldering conflict that doesn’t do much to change the current status quo but keeps Russia and Europe at odds, which the US wants anyway. I think this is just a signal from Putin that if the US goes all in, he won’t hesitate to bring in Russian troops and permanently sieze control of that region.