Yep, but the trick is in his delivery. If his lips were moving it was a lie, but to break it down, he probably hadn't had sex with Helen Thomas. Mentioning Monica was a non-sequitur. That way, despite giving entirely the wrong impression (that he was saying something else), he could say he told the truth. It left connecting the two statements to the viewer, and for the viewer to see a denial that wasn't actually there.
Ultimately, I think he came down with "Well, I didn't have sexual relations with her, she just had them with me." Something like that.