By definition lying is stating something that the speaker knows to be untrue

No. It is true that a lie is something one knows to be untrue. And lying is defined as the present participle of lie. But lying is additionally defined as "not telling the truth". It turns out words can have multiple meanings, and the latter definition is not dependent on the speaker being aware of its untruthfulness.

If we were talking about speakers telling lies, there would be merit to what you say, but since we are talking about lying, that is not a necessary precondition. By definition, not telling the truth, even if erroneously, is, in fact, lying – although it is not telling a lie.