Clever! You got the point.From a pure game mechanic it makes sense (to me at least) and gives one more advantage to being invisible (defended tiles cannot be pillaged without first killing the defender guarding the tile AND repelling an initial offensive). The case that needs to be considered is if multiple units are within range of pillaging; can the subsequent units attack and kill the defender or are any attacks against the unit aborted until the next round. Is being able to infinitely hold off a pillager with fodder likely and over-powering (since you could bring in another unit - or hide the now revealed defender - to guard the tile and repeat the process; the subsequent attackers question is necessary to resolve before addressing this)
Being invisible should be an advantage, but defending must work for all fighting units.
In most case when moving or fighting - here defending - an unit become visible to the intruder. But only AFTER the defending action will be advantagous.
IMO the easisest way is a good, reasonable defending or hiding bonus. E.g. the fight probability is increased by let's say 25% or two first strikes for the defender.
Otherwise most players will avoid Nox noctis - just read Sarisin's comment.