Can someone explain to me how ranked hidden MMR makes sense?

So please correct me if I'm wrong at any point in my understanding of how the ranked matchmaking system works.

The system doesn't care what rank you are when choosing teams, instead it cares about the total team hidden MMR, and to a lesser extent (time permitting) the difference between players, so it's less likely to pit a platinum with a bunch of bronzes against silvers. This works to create balanced matches at the cost of potentially forcing players of a much lower tier to experience matches at much higher ratings.

If I understand correctly however, losing one of these matches will have a very small effect on your LP, while winning one will have a much larger effect (assuming your hidden mmr is higher than the tier you are in). This type of matchmaking occurs even in your promotionals, which means your chances of losing are equal to your chances of winning. In other words the chances of you getting out of your tier/division are based on how skilled the game thinks you are, not the skill required to exit that tier/division.

So I 'kind of' understand why the hidden mmr matches you with more difficult opponents, but shouldn't the promotionals be as difficult as would be required to exit that division?