Yes, that's true, but the point, I think, is that Dale needs to walk in prepared up front openly how we went wrong and how he really feels. Once this has been started, this is an invitation for Marigold to open up in turn and discuss her own feelings and her jealousy, and how they can address that (which would, I assume, come down to addressing her insecurity).
Dale walking in and immediately discussing Marigold's shortcomings will make her defensive, multiply her insecurities, and in the words of May, make things 1000 times worse.
If there's one thing I've learned in my relationship, is that after a fight (and after some time to cool off and to regain the ability to think clearly), the best thing you can do is to openly confess shortcomings, however minor they may seem (in reality, probably not). This is infinitely more likely to result in your partner reciprocating, whereas shooting accusations and demanding changes in behaviour will just result in barriers going up.
This is a slight side note, but I think that one of the major mistakes Marten made back then was to respond to Dora's jealousy with anger rather than with understanding. This meant Dora would have been hesitant to discuss her jealousy when it arose again, and it would only emerge as another explosive fight. If there had been support and reassurance... well, I'm not saying things would have been different necessarily, but the odds would have been better.
This isn't about laying blame, by the way, before anyone says that. It's just an observation.