You probably mean a filter with "thick paper" (carton in french, not sure how to translate this)
Well yeah, a rolled up peice of thin cardboard, or a chunk cut off a pack of smokes, that cardboard works good and easy to roll up without going fuzzy. Basically all it does is keep the chunks inside the joint and keeps the opening open. It's not really a filter, although it does get gungy with resin at the end, it's pretty much useless for cleaning the smoke you inhale, therefore there is less filtered smoke going into your lungs in a joint compared to a cig. Esspecially if the join contains tobacco mixed in with it. That tobacco you normally would have inhaled with a normal filter, is now going directly into your mouth and lungs, which would conclude joints would be more harmful to you then a cig.
But once again, I have yet to see a study on weed alone, ie: a bong/pipe hit compared to a cigg. You usually smoke a lot less in a bong/pipe then you normally would in a joint, as you can put in as little or as much as you want.
Perhaps that is also why they compared to joints rather then weed via bong/pipe, as it's easier to measure in comparison. But then I'm stuck on the "Did the average joint in each comparison contain weed only or with tobacco?"