Currently, the rule as written in Arena and Domination is that a Guard protects everything in the zone, including neutral and enemy objects, from attacks by your enemies. I've actually advocated for this being changed, and Arcane Wonders did make the rule in Academy that guards only protect friendly objects, so I am hopeful that it will be updated at some time in the future. Such an update has not happened yet though, and might never happen, so you played correctly as is.