This is a fair question. I haven't discussed this answer with Mithical, but I'll give you my perspective.
The Codidact project aims to do better (much better!) than Stack Exchange in some important ways, most notably how we serve and involve the community. I think it's fair to say that Stack Exchange Inc, as a for-profit company with a poor track record on the community front, does not value the communities it hosts and will run roughshod over them whenever it's convenient. (Even now, we see that the company does not follow the agreement it imposed on moderators. Commitments run in only one direction there.)
Thus, I would be very concerned if an SE employee were in a leadership position at Codidact; even if the individual has top-notch ethics, that's a tough situation to navigate and there's at the very least a strong appearance of a conflict of interest. If an employee there were interested in a leadership position here, we'd all have long, difficult conversations about it.
But moderators and community members aren't employees.
I think most moderators, and this was my experience when I was an SE mod, are there for their communities, and despite the shenanigans from the company. Personally, I cared deeply about Mi Yodeya in particular, and I was prepared to continue serving that community despite problems from the company. For years the company ignored most of the smaller communities, and we navigated that neglect just fine. We would have continued to do so if they had continued to ignore us and just left us alone. Alas, that did not happen. But I can completely understand the perspective of the other long-term community members who built Mi Yodeya and weren't personally targeted by the company. Would I like more of them to come here? Yes, of course. Do I understand why sunk costs and an existing community (even much diminished from 2019) keep them there? Yes. The end of SE communities will be a long slow slide into decay and attrition, not an inflection point where everyone looks around and says "we need to get away from this company".
I suspect that Codidact directors who remain moderators on small communities there are experiencing something similar: despite the evils coming from the company, they still have communities of people they care about there. Many SE moderators are currently on strike due to a new round of destructive company activities, and even so, many of them are trying to fix it rather than leaving. It's hard for people to walk away even when, objectively, they know the ship is sinking. It was hard for me to leave. And as we've already seen, communities don't migrate en masse; I'd love it if whole communities there would pick up and move here, but that's not the usual pattern. Even Writing, the community most damaged by SE's actions a few years ago, didn't migrate.
So let me say some things to, I hope, allay your concerns.
First, I trust my fellow directors to do what is best for Codidact, regardless of involvement elsewhere. It doesn't have to be either/or; it can be both/and. I've gotten to know Mithical over the last four years and I have no concerns.
Second, all members of the board of directors of the Codidact Foundation are legally obligated to work to the benefit of Codidact and declare any conflicts of interest. One of the reasons we formally incorporated is to be able to make that commitment. You don't just have to take our word for it; if we go astray and turn evil, the UK government is going to smack us.
Third, we are playing the long game here. Our communities won't have explosive growth overnight. It's going to take us a while to gain traction with search engines, with the open-source community, and with all the people out there who want a place to ask questions and share knowledge with like-minded people. It's going to take even longer if those people have, or think they have, a home now -- on SE or Reddit or wherever. We continue to build something better, to work together with our communities, to help people who want to use our software but not our network, and to make our corner of the Internet a better place that puts people first. Every Codidact director shares these goals.
Codidact isn't "SE 3.0"; it is its own thing. We hope to attract people from SE, and also from many other places. SE is in decline and that will help us eventually, but even if SE Inc. were to magically get its act together tomorrow, we'd continue our work here because Codidact is intrinsically valuable.