Prior to joining google in 1999 as employee number 20 "Mayer worked at the UBS research lab (Ubilab) in Zurich, Switzerland, and at SRI International in Menlo Park, California" (wikipedia) for what appears to be only a few years. (She graduated high school in 1993, received a B.S. and and M.S. which must have taken at least 4 or 5 years afaik).
My guess is that she has a great deal of confidence, confidence built on the fact that she was an early google employee and she was able to ride the success of that opportunity. But her entire working career is basically google and I'm not sure she brings the depth of experience necessary to take on turning around the fortunes of Yahoo.