Yep, kdenlive is a Plasma5 package and plasma6-kdenlive is the one for Plasma6. And also yes the dependencies for each would be different.
I understand this is confusing for new user and a lot of longer term users as well. But wait! In Cooker the Plasma5 packages are gone and the plasma6-something naming is being reverted to drop the plasma6. This is a big reason why we are still waiting for the next big upgrade for rolling (ROME) repositories. There are some other big changes as well.
And I will probably forget this at least 50 times… 