With Time Travel, if our descendants visit us, would they be ‘aliens’?

so, an antideluvean civilization of high technology, invents near light speed [relativistic] travel, the time dilation results in a trip to and fro, returning many thousand years later to a degenerate civilization [us] that still thinks smartphones are a neat idea.

How could a people with only sticks and clubs* for weapons possibly invent time travel?

* (Einstein)