So the law says attacking in a city without you or another imperial being attacked first is bad.
In this particular case a skill that does not require you to be in the same area is being used.
If we assume that the victim recalled to a protected city we have a situation where the victim is in a city and the attacker is not.
This is where mortal interpretation of law comes into play. This is a gray area because you have to determine if the law is based on the location of the victim or the attacker, since both are not in the same place.
This kind of distinction is generally left up to the mortal leadership. There have been times where the interpretation of the law was so lenient as to allow the attack in a city if the non imperial started the fight and retreated to a city during the bloody timer.
Conversely you could be ultra strict and determine that doing any harm, even with distance abilities like eye for an eye, unerring smite, and night gaunt are not cool.
The actual "line in the sand" for those calls fall to mortal leadership. Because it was the emperor in this example it's clear where they draw the line.
Because empire is orderly they need to apply the laws consistently so as long as they don't bust other imperials chops for the same behavior everything is fine.