That's simply the most recent in a long line of changes. I remember in my own lifetime when it was declared that no mammals lived during the Mesozoic Era, period! Then it was the little ones. Now it is bigger, more aggressive ones...
I will grant you that I do not expect mammals of any sort to live in the steamy river valleys and other warm geological areas which were more comfortable to those big reptiles, but that they lived at the same time I do not doubt.
We certainly have evidence that men were aware of dinosaurs living at the same time they did. The Chinese had their dragons, Alexander the Great's men record being terrified by some kind of monster in a cave, old epics such as Gilgamesh and Beowulf all indicate man's knowledge of 'monsters' that we do not know of as being alive today (although some are still searching!).
Mammals and dinosaurs at the same time should not be a shock to anyone whose investment in the standard evolutionary theory is not set in concrete!