I think people are referring to the idea that pot is not physically addicting, meaning that your body won't experience withdrawal symptoms when you come down.
Tons of people are habitual smokers though bordering on mental addiction.
I now deeply believe (I'll admit I have no more evidence than my subjective experience) that many of our abusive substance intakes, be it cigarettes, alcohol, weed or even coffee or sugar, are in part to cope with those unpleasant physical sensations, not just the mental burden. Slowly, you build a physical intolerance to them because you are used to be liberated from them using whatever is substance you abuse.
And hence I do think there is a physical addiction as well.
E.G: if you can't cope with stress without getting some THC, I don't classify that as only mental, as stress is not a phenomenon that is only in the mind, but affect the entire nervous and hormonal system.
I think there's another class of substances though where withdrawal results in symptoms like fever, vomiting, etc. that are consistent across users regardless of their mental state.