Yes, exactly correct. Having the true throuple route go through a committed monogamous relationship makes no sense. It would have been better if it didn't exist at all, and for the open relationship to simply fail when the girls realize they want to command MC's full devotion. At least that would have been intuitive for their characters if the devs wanted to go for realism.
But what makes no sense to me is that Mallory is clearly ok with sharing MC with Amanda. If Jaye is supposedly getting as close to her as her lifelong friend, why couldn't that precedent extend to their open relationship? Similarly, Jaye is ok sharing with (and being with) Tara, so it seems mutual.