Matt Lauer is in a difficult postion. He is expected to only report on the truth, but at the same time he is expected to remain nuetral and alllow his subjects to speak for themselves. Calling Trump out, particularly if he continued to deny it would be seen as bias, because he had little proof.
Matt Lauer's job as forum moderator for the US presidential candidates is to ask the hard questions so that the US public can get as accurate a picture of each candidate as possible. As a moderator, Matt Lauer's role was that of a journalist, therefore, if he brought up any inconsistencies and outright lies stated previously by Donald Trump regarding the war in Iraq, it would have served the purpose of transparency. If Donald Trump was inconsistent regarding his stance on the US's invasion of Iraq, the American public should be well aware of this matter prior to Election Day.
No, calling out Trump more forcefully would not have been seen as bias. Matt Lauer should have presented the story with more force if it was warranted so that the American people could have gotten the right impression. It is very important for voters to know very clearly if Trump has been lying on big issues.
Matt Lauer would not have been seen as bias had he called out Donald Trump about his lies regarding Iraq and Libya. A journalist has a right and obligation to forcefully question his subject. This is particularly so if he is perpetuating lies. This would have given Lauer more credibility with the public.