A Line In The Sand - Why was it necessary to lie?

During the 9 October 2016 debate Donald Trump stated, “First of all, she's there with the so-called line in the sand.”  Hillary Clinton responded, “No, I wasn't. I was gone. I hate to interrupt you but at some point we needed to do some fact checking.”  Trump was obviously referring to Obama’s statement about Assad crossing a “red line” with the use of chemical weapons.  Hillary Clinton was States Secretary of State from January 21, 2009 until February 1, 2013.  The President made his “red line” statement in August 2012. Why was it necessary to lie?

The August 2012 statement

“I have, at this point, not ordered military engagement in the situation.  But the point that you made about chemical and biological weapons is critical.  That’s an issue that doesn’t just concern Syria; it concerns our close allies in the region, including Israel.  It concerns us.  We cannot have a situation where chemical or biological weapons are falling into the hands of the wrong people.

“We have been very clear to the Assad regime, but also to other players on the ground, that a red line for us is we start seeing a whole bunch of chemical weapons moving around or being utilized.  That would change my calculus.  That would change my equation.”“I have, at this point, not ordered military engagement in the situation.  But the point that you made about chemical and biological weapons is critical.  That’s an issue that doesn’t just concern Syria; it concerns our close allies in the region, including Israel.  It concerns us.  We cannot have a situation where chemical or biological weapons are falling into the hands of the wrong people.”