I think we are a little more on track here this time.
If travelling produces time dilation from another frame of reference, it is only because it is measure from that other frame of reference in the first place.
As far as the physics is concerned, the traveller is not moving thru time any differently than we are. If you want to say that daily life, growing older, and progressing toward the future in the normal course activity is "time travel", OK. The we have a semantical difference. I don't call this time travel since we are never "travelling" thru or to another time. It's just the never ending procession of daily cause and effect, which is no different from what the traveller does. Even in our very same "timeline".
I chose the example of him remaining in sight of the earth since it clarifies the perspective for some a little better. It's no different if he goes far away to another galaxy even.
If he was gone a million earth years, and he travelled fast enough to make a return trip in HIS lifetime, he would still be a million "years" old. He just would not have aged that much and his watch would not have ticked off as many seconds.
Carl Sagan once calculated that at 99.9999 percent of the speed of light, an astronaught could traverse the known universe in about 54 "years" from HIS point of view.
The Earth would have long since been swallowed by the Sun and the Sun died out to brown dwarfism.
He would not have travelled thru time. He progressed along the same timeline as you and me, having been fortunate enough to outlive us all due to the effects of relativity on his acceleration and velocity. Thats all.
Maybe not so fortunate since all his friends would be long gone, and maybe even the human race if we did not colonize while he was away.
Your first post implied there was a technological solution to "time travelling" this way when we develop it, but it's really not the issue since it is not time travel in the first place. It's an effect of relativity, measurable only as a difference between frames of reference.
In the case of the guy who was gone 50 earth years but aged only 5, upon returning to earth, he would be 45 "years" younger (in aging) than his brother, but still the SAME number of years (earth revs around the sun) elapsed for both. Perhaps I should say philosophically, he travelled to the past not the future, since the remaining twin is definetely "ahead" of him.
They are both still the same age in actual years. Exactly. It's just that the travellers biological clock, as well as his wristwatch slowed down due to the dilation effect of relativity. It is not travel to the future, it is a remedy for aging.
It's a conceptual issue, not a "time travelling" in any sense at all.
Peace.