So was it ever stated that Rowan just got very very unlucky when he failed to behead Jezera or did she had endurance and regeneration that would make every attack he did worthless?
I think it boiled down to that regular demons (not demon lords) can still be killed by regular weapons, but spawn of the demon lord needed magic weapons to take down, which Rowan didn't own because he was a peasant, not a magical knight.
Edit: I think its still technically POSSIBLE to kill them without those, just really really hard since both of them regenerate when damaged, as seen when Andras gets hit by an arrow during the siege and the wound is gone minutes later.