The elements of a "conspiracy" are 2 or more people conspire to commit a crime, the defendant willfully becomes a member of the conspiracy, and that one of the conspirators knowingly commits an overt act to further the conspiracy.
I can understand why you would think that it is a "loose" legal phrase because it is kind of a catch-all kind of crime. But, in many cases, the government has proof that a defendant was willfully involved. It doesn't matter that he didn't commit the "overt" act as long as someone else in the conspiracy did.
In my jurisdiction, you hardly ever see conspiracies involving violent crimes (i.e. robberies, kidnappings, etc.). You see many more drug conspiracies where there is a group of individuals that deal drugs. Unfortunately, the low level dealer can get caught up in it and face just as much time as the top guy.
My guess is that they offered Pollard 2 years of diversion on such a serious case because they didn't think he was very involved. Just looking from the outside.