Whether a person was convicted of a specific crime is a fact. It's true or false. People can disagree about whether the trial was fair, the law was just, etc... but there is an objective truth as to whether the conviction occurred or it did not.
The Snopes article lists the crimes Rosenberg was convicted of, which do not include "terrorism" or anything equivalent to that. She was convicted of multiple offenses involving weapons possession and identity fraud. The article goes on to list numerous other offenses involving a prison break, several armed robberies, and planting bombs for which she was charged, but not tried or convicted. It is therefore correct to say that she is not a convicted terrorist.
The article provides more than enough background for a reader to form the opinion that her activities constituted terrorism, and indeed that is my opinion.