Tenuous at best. What they did was an act of war. You could argue that striking back, even against civilian "strategic" targets was an acceptable response. However, I suppose I'm a lot more specific in what I consider terrorism than some.
The Russians intervened to support a friendly government -- puppet or not -- and were there with its consent.
You know that drill, don't you?
One man's freedom fighter is another's terrorist...
Interestingly the same mistakes are made over and over. The Russians went in to prop up the Socialist secular regime that was dependent on the Soviet Union but under attack from the Islamists and in danger of collapse.
The Russians expected to support the Afghan army and allow the Afghans to do the bulk of the fighting, but the Afghans were not capable of it and the Russians were sucked in ever deeper.