I belive true/false is considered by it's numeric equivilents (0/1) so this entry:
<decision_entry>
<!--
if human, fortified defense
-->
<min_entry num_settlements="1"/>
<max_entry target_human="true"/>
<faction_attitude defense="defend_fortified"/>
</decision_entry>
True > false so it is always returning a true argument, and not continuing. Try entering a min_entry target_human or commenting this argument out compleatly and test. I'd also continue this argument as 'fortified' against a non-neighbour is as confusing for the AI as 'frontline.'