On the one hand, if you grew up in the baazzar, moving to the cathedral might feel like the "death of open source" even if it is really just a return to an earlier way of working. On the other hand, while not accepting external code contributions will certainly improve their security posture it will also make it more difficult to identify who to invite to join the priesthood.
So I do not see a problem with Ladybirds decision, in contrary, IMHO it strengthens the human aspect of software development and puts the brakes on AI free riders