Manifesto
From the very beginning, the story has been characterized by sudden reversals, deliberate deceit, and unreliable narrators. The reason for these tactics is to mystify the viewer. Therefore, why should we believe any of the events seen in flashback in the final episode?
Never mind what Tom Rob Smith says. He has fooled us and lied to us throughout the series. We will not be deceived again.
The truth is that Alex is alive.