Like I said, that's only true from the COLONISTS' perspective.
It depends on which time travel philosophy they're following in this show (which is something we don't know - maybe the writers don't know either, wouldn't be the first time that's happened

).
Under one rule of time travel, there is no future to any given timestream. It is created by the actions of people living in the "present," and until they act, there is nothing "ahead" of them. It's all just a big blank canvas. Their actions
create what is ahead of them. Time travel from the future isn't possible because there are no people yet to travel back in time. Travel from other realities is still possible since those people have already been created by their timeline, which presumably is "going faster" or is offset in some way so it seems to be ahead of the other. Maybe the Big Bang happened earlier in that reality. So it's not time travel at all, although it could appear to be if the realities are similar enough.
Under another rule, each action creates a new branching timeline, and the infinite number of timelines created encompass every possible permutation of reality that is compatible with physics. If you mess up one timeline, that's okay. It's certain that everything will be happening like "it should" in another.
Under yet another rule, the entire timeline has already been determined and nobody's actions can change anything. Free will is an illusion; everyone is just enacting a pre-determined script. Under this rule, time travel can only happen if it was always part of the timeline anyway. Not only is there no danger to time travel, it can't possibly hurt anything. If you can time travel at all, you are required to.
And there are many other rules, such as the notion that time travel does overwrite the future, and if there are time travellers in our past, our own timeline could be being overwritten right now. It could be overwritten a million times per second. We'd have no way of knowing. At any given moment, we think "this is the way it's always been" when in fact, there
is no way "it's always been" and we exist in an eternal state of oblivious chaos.