I'm not totally sure, but I think their mainboard is of their own design. They would need to adapt, but they could do it. I also think the differences are not too large, since most mainboard manufacturers offer surprisingly similar mainboards for either brand.
The modularity of some components can be assumed because they are industry standards, like wifi modules I suppose. Other components perhaps Framework have designed their own range of modules with a common form factor, but it must be very expensive to engineer a compatible mainboard in the same form factor with a different chipset, unless they are using an existing standardised design.