OK, so the point is? Would the scientists employ their time travel resources on missions that are supported by the experience and evidence they have at hand, or would they employ them any number of other ways, on the principal that they don't know everything?
The point is that just because the character was told time travel was impossible, that doesn't absolutely have to mean that it's impossible. Sometimes characters in a story, even scientists, are wrong about how things work. Case in point: Source Code
. That was also a movie where the scientists told the hero that he couldn't change the past he visited, but the whole plot of the movie was about the hero discovering that the scientists were wrong about what was really happening and how it worked.