Clinton's comment is intended to play to the gullible masses. She knows law enforcement will not engage in argumentation with her, thus, it will appear to some that she's made some point for which law enforcement has no counter.
Law enforcement and the courts know a violation would have occurred irrespective of how an email is marked. It's the classified info contained, not simply whether it's marked as secret or had such a physical designation removed by the time she sent it. Removing such a designation is also a violation, yet apparently happened a lot while such documents were in her care and that of her staff.
HF
What I haven’t read anything about is whether these emails were the 1) original ones which had to remain on secure government servers. Or whether they were 2) summaries or “copy and paste” versions which were sent to her (outside the secure government server) by State Dept. staffers.
Either way, what was done was a violation of the law. But they may have thought by doing 2), there was deniability.