Wrong, if there is a run in need of your class that is in progress and you mark the join in progress box you will get that first.

I have joined a run in progress plenty of times as a healer when a FC mate calls for help because their healer left them, I que up and mark join in progress and always get in their group.

Also another note, if someone drops from the run you can leave without incurring the penalty. You only get the penalty if you leave a full group.