Ad-free channel or HD only channel doesn't mean premium channel.
When a channel has some exclusive content to show on al-a-carte basis only... then it can be termed as premium.
Like SWP HD is a premium channel though it shows ads. They have exclusive international studios content with nearly same time as it broadcasts in other countries. This channel only available as al-a-carte.
SMS HD is more likely MN+ as it should be available in some base packs.