They could have, I doubt it but they could have, yes. One was forced by her father to put her baby up for adoption when she was a teenager (back in the 50s). She never had any others, and she was against abortion. Two others were my great aunts, one who died in the 50s and one who had a mastectomy at age 80 a few years ago.
How do they know then, that the women in the study who say they never had an abortion are telling the truth?
If you read the study, this will be a bit more clear. The statistics were gathered in countries in which the abortion statistics are thought to be more complete. The US and France were left out because abortions tend to be under-reported. Reading between the lines, they studied countries with more socialized medical systems and less medical privacy. No doubt there are errors, omissions, and additional factors which could be influencing the outcome. This isn't an easy subject to study. There can be a long induction time for cancer, and there are a lot of political sensitivities and ethical considerations. I'm glad to see someone trying to get a handle on this connection, difficult though it may be.