They meant that it's logically impossible for the probability of A AND B to be higher than the probability of A, since A AND B is a subset of A. She could be feminist, but the probability of being feminist is lower than the 100% probability that she's either a feminist or not.
Overall, I find it rather ironic to find such gross errors in an article that sets out to educate the reader about "human error".
Then they went around asking people the same question:
Which alternative is more probable?
(1) Linda is a bank teller.
(2) Linda is a bank teller and is active in the feminist movement.
The vast majority—roughly 85 percent—of the people they asked opted for No. 2, even though No. 2 is logically impossible. (If No. 2 is true, so is No. 1.)
They say: "If No. 2 is true, so is No. 1.", but they omit that the opposite does not hold. Perhaps they thought it was obvious enough.
I think they get it and just left that second part out. They may have worded it poorly (better would be "it is logically impossible that #2 could be the correct answer"), but it was very clear to me when reading the article that the editors meant exactly what you describe.