( and don't listen to the doomsayers bashing altium. they are just jaleaous they have to muck about with counterintuitive, counterproductive, stone-age tools and can't afford something like Altium)
I have an Altium seat and subscription. Maybe some 'doomsayers' base their opinion on past performance, maybe some bought Altium stock - lol.
This castellated PCB modules on a carrier board smells like another of Altium's hair-brain schemes. They probably have visions of selling modules (possibly with firmware IP support) in their content store. Situs can probably manage to route the interconnect - lol.
They will spend a lot of money providing seamless support for creating systems from these building block modules and you will be able to design your Wifi enabled spirit level or whatever in 15 minutes. Which is great if your business is selling expensive Wifi enabled spirit levels. The real world will look at it, maybe try it and then mostly ignore it because there won't be a set of modules to do what their business is and the product will be expensive unless they have little volume and the module suppliers (and to some extent Altium) would have them by the balls.
I and I suspect most Altium customers would rather they spent our subscription money improving the core functions of the package.