NBC originally reported the insult, and Tillerson was asked about it by a reporter immediately after that. Tillerson evaded the question. Later, a surrogate was trotted out to deny it.
Now, Tillerson just appeared on CNN and was asked again. Again he refused to answer, bloviating for several sentences about not "dignifying" the question by answering it, and other high-minded blather. A simple "no" would have put the matter to rest.
Apparently Tillerson has some kind of internal moral code that prevents him from telling a bald-faced lie, so instead he squirms and twists and implies a lie, denying without explicitly denying. Tillerson was an Eagle Scout as a boy, the top level in the Boy Scouts. That organization indoctrinates boys with a moralistic ideology. This may be a factor in Tillerson's unwillingness to flat-out lie, like his boss does as naturally as he breathes.
Trump, for his part, challenged Tillerson to an IQ test competition. No word on who Trump would appoint to officiate. Maybe Ivanka or Jared Kushner or Donald Jr.