When I first tried upgrading outposts with a Warden upgrade, it did not seem to stop them being destroyed by monsters. Today I was looking at the xml, and saw this:

    <ImprovementType InternalName="Outpost_Warden">
        <DisplayName>Warden</DisplayName>
        <Description>Most monsters won't enter the outpost's Zone of Control.</Description>
        <HideInBuildList>1</HideInBuildList>
        <IsOutpostUpgrade>1</IsOutpostUpgrade>
        <Prereq>
            <Type>Tech</Type>
            <Attribute>Recruiting</Attribute>
        </Prereq>
        <LaborToBuild>150</LaborToBuild>
        <ArtDef>Art_Outpost_Warden</ArtDef>
    </ImprovementType>

Note the lack of a <GameModifier> section. I think the Warden might genuinely just not do anything at all.