The November 28, 2013 Supplement has clarified this section of the rules. It now reads:
"If there is an enemy Guard (a creature with a guard marker) in a zone, then you cannot melee attack any object in that zone other than enemy Guards. This condition is checked when the attack is declared."
That, and also in the german rules it says (translated): "If a creature is in a zone with one or more enemy creatures that are guarding, it may not make a melee attack against a
creature without a guard marker. Exceptions are guards it can ignore, see sidebar."
I am part of the german Mage Wars realisation team (I am lector) and I know that our responsible persons do confer with the US team before the rules are printed, thus I take this as a hint how the rule might have been meant in the first place.
Im pretty sure the reason why this is in the FAQ is to solve the problems with guards not protecting conjurations from flying creatures.
There are a few badly worded clarifications in the FAQ that and this could be one of them.
I put my money on guards will protect walls still. (obviously only if the attack occurs in the zone that the guard is in.)
Because of what I have posted above I would gladly accept the bet and say guards do only guard creatures in their own zone but I think we will have to wait till we get an "official" answer by someone of the US Mage Wars team.