I think it is, because "Transfer trade" is an available action you can ask for outside of peace deals. "War reparations" is not available.
But still I think Trade power via peace deal shouldn't cost you a dip slot. I think it should be the same for Fleet basing rights and Military access. The dip relation cap is just the number of relations your diplomats are able to maintain. If you enforce something via peace deal, your diplomats don't have to do anything in the next 10? years. And yes, it makes still sense, that vassals etc. should cost the slot, because they don't end after a few years and you actively have to maintain them.