It's the streamer who decides when the ads should show (this is because those ads hide the stream, so they should be used when nothing is happening). If they never tell Twitch to show ads, SureStream ads won't appear.
I watch at least one streamer who runs ads in her breaks, she is a partner. So does she have a to specifically select those surestream ads? Maybe they only show in certain geographic locations?
Are those special and only run for large streamers? I never see ads on twitch, but the biggest streamer I watch has under 300 viewers.