But they apparently had been doing it for some time and would have had firsthand experience and data.
No remotely honest scientist would ever claim to know everything about their work. That's just not how knowledge works. We've been traveling in space for half a century, on and off, and we're still learning new stuff about the effects of space on the human body, developing new types of propulsion and other space technologies, etc. We've had flight for over a century, but aviation engineers are still figuring out new things about the physics of flight and how to improve performance. Discovery is not something that stops.
And maybe there are different levels of time-travel technology, just like spaceflight is a level above aviation or interstellar flight is a level above interplanetary. Each new level requires new technologies and methods. So maybe there's a second level of time travel, a nonlinear method that the scientists in the movie haven't discovered yet.