Maybe I’m missing some inside baseball, but what exactly is wrong here? Dorsey created Frankenstein and has regrets. I don’t see what’s childish about that.
He wanted to change the company in ways that he thought were better, but he couldn't get the support of the other stakeholders so he's leaving the board. He's discussing with a friend a combination of A) lessons learned and what he'd do differently if he could go back with his current knowledge and B) an abstract hypothetical that after he leaves the board maybe he'll work on a new project that does things better.
There is a huge difference between "I'm going to use my position at company X to harm company X" and "After I leave company X, I'd like to do a new project using the lessons I've learned".
It appears, not only from this text but many other comments he's made, that he found the board to be just as difficult to work with. I think he's spot on here, identifying the root cause of the problem, and a realistic appraisal of the chances of changing the existing company.