The article does not state that there is no intelligent life elsewhere, but that the chance that it exists has become a lot smaller: "The kind of merger that creates mitochondria seems to be a ludicrously unlikely event. Prokaryotes have only managed it once in more than 3 billion years, despite coming into contact with each other all the time."
If the universe is infinite, and the conditions are the same everywhere, then intelligent life should just be there, but very rare.
Jack Vance (science fiction writer) once said that In a situation of infinity every possible condition occurs, not once, but an infinite number of times.
Whether we will ever know, I am not sure. In principle, any speed below the speed of light is possible (provided that all problems that arise at such a speed can be solved), and then it doesn't necessarily take very long to reach far away solar systems, it will only be annoying for the travelers when they return to find out that millions of years have gone by on earth.