If a user is an editor of a channel, they can access the dashboard at twitch.tv/channel/dashboard, and start a commercial from there via the commercial button. However, that same user, despite being an editor of the channel and despite being able to run commercials from the dashboard, cannot use the /commercial IRC command. (JTV response: “Only the owner of this channel can use the /commercial command.”)
Correct, the IRC command only works for the broadcaster. Editors can only initiate commercials via the API and website.