Time travel is impossible because it assumes:
A: We have no free will and all things are predestined.
B: If all things are predestined, then they can't be changed.
So to make time travel possible makes itself impossible.
The only thing that is possible is to observe the past or the future but not interact with it. The Thalans in this case could see what was happening but could do nothing about it.
Now if the universe is not predestined, then going back in time, the timeline would be completely different when you get there. Say 2000 years ago in your history there was a great war between two competing civilisations. You go back in time and they are best friends. Your thinking then wouldn't that change my history? No because the universe has no memory.
The universe is only about probabilities and the further in the past or future you go, the more varient it becomes from your expectations. Time flows in both directions from point zero or this "instance". Think of it as time flows away from you in all directions. This means that if you only go back in time or into the future by a small degree it will be highly like your current universe. Going in time by a great deal would make it highly unlike your current existence.
The amount of "drift" from time zero is anybodies guess. The current theory of time travel assumes a drift=0 scenario but that is not the only possibility and if time travel (interaction + observation) is possible, then drift > 0 is the logical conclusion.
In this case, even if the Thalans stop the humans in the past, it is a futile exercise because it won't change their time zero. They will be destroyed, the humans will be destroyed. In fact, destroying the humans in their past might create the very event they wish to avoid! Wrap your mind around that one.