I think that each sub community has to decide this independently.
Codidact's vision was created with the bad experiences on SE so naturally there are a lot of users who were (or are) regulars there. It's highly likely that now still many if not all users know Stack Exchange and participated somewhere in the network.
I for one am happy that we didn't import anything for the site where I contribute the most (which is Cooking) as I have no affiliation with the original site and didn't provide anything there. So a blank space offered me and anyone else to contribute from the start while knowing that questions and answers originate from Codidact. We therefore build up this community from scratch.
On the other hand, we have communities where power users have migrated here as well (Scientific Speculation, Judaism) so they do have a legitimate interest to import questions and answers as they have poured in significant time and work. As they know a lot of the imported content, they can navigate it, knowing somewhat precisely what's has already been asked and answered. So while these communities start over from scratch too, their situations are different as they bring in knowledge.
Basically, this less a general policy question and more a community question with policy-related effects. Communities should nevertheless carefully evaluate if they want to truly start from scratch or bring in existing knowledge and content (and be confronted with the mentioned effects).