Since the RR reduction is only for high elo, specially for the Radiant rank, you can think of this 5 stack queue almost as a different mode completly but inside the regular soloQ. I think they want to focus on people that want the full team experience in Valorant without the need to search for another 5 players. With this update they can essentially scrim against each other without really gaining or loosing much, but if you do win, you get a little reward in LP. Teams will get a way better opportunity to train and rank up while doing so, but not by a shit ton that would impact your soloQ mmr, which honestly is a totally different kind of game.

In the past riot tried to introduce Ranked Teams or 5 stack queue only in LoL and in the end those queues never really did get much interest from the player base as a proof of skill. SoloQ always have been the main queue people focus, so doing this the way they are incentivazes people to queue as 5 because it is integrated inside the same queue, instead of in LoL which no serious team really plays on the 5 stack queues.