They never "breached" Stormwind, what? All they did was sneak into Stormwind at night, which an army obviously can't do.
Following your logic, Tyrion and Davos also "breached" King's Landing when they infiltrated the city in S7.
Infiltrating a city with a few other people =/= Breaching a city during a siege
Also: 1) they didn't set fire to the entire city, only to the buildings in front of the harbour and 2) in the CGI 8.1 cinematic, where Anduin frees Saurfang, said buildings were shown to be in a perfect state, meaning that whatever damage Zul did was irrelevant and quickly fixed.