There’s a difference between “technology that we don’t know how to do but it’s fine in theory”, “technology that we don’t even know if it’s possible in principle” and “technology that we believe isn’t possible at all”. Uploading humans is the former; we have a good theoretical model for how to do it and we know physics allows objects with human brain level computing power.
Time travel is the latter.
It’s perfectly reasonable for a civilisation to estimate that problems of the first type will be solved without becoming thereby committed to believing in time travel. Being ignorant of a technology isn’t the same as being ignorant of the limits of physics.
There’s a difference between “technology that we don’t know how to do but it’s fine in theory”, “technology that we don’t even know if it’s possible in principle” and “technology that we believe isn’t possible at all”. Uploading humans is the former; we have a good theoretical model for how to do it and we know physics allows objects with human brain level computing power.
Time travel is the latter.
It’s perfectly reasonable for a civilisation to estimate that problems of the first type will be solved without becoming thereby committed to believing in time travel. Being ignorant of a technology isn’t the same as being ignorant of the limits of physics.