Strictly speaking, barriers don't prevent the effect from kicking in. But I've had people occasionally survive with minimal HP. The interface doesn't handle that well so it doesn't update properly, so I'd move ahead to next turn, notice something's off, then hit reload to see 'oh, <insert hime> is still alive?'. So something was up, but I couldn't quite be sure what.
In today's run, I had the effect land on Sol and proc with a Tiamat barrier up. I reloaded and saw that Sol survived with exactly 750 HP remaining. So I'm suspecting that the instant kill effect is not so much 'set HP = 0' or something like that, but more a 'inflict 100% HP damage' that functions similarly to the other 'inflict X% HP damage' stuff we've seen.