The split makes sense for practical reasons - I also sense he is personally perhaps more passionate about browser hacking than OS hacking (his own contributions were more to Ladybird than to the OS for about a year as he himself writes). Smart as he is, he may have recognized that he is in a unique position to be able to contribute a cross-platform browser that competes with the big tech companies, where as SerenityOS is essentially more of a toy OS (32 bit, 1990s look and feel, not compatible with important other operating systems, no radically new OS concepts) - without wanting to dimish the contributions of its amazing developers. IMHO, SerenityOS is more about the process of writing code from scratch than the resulting software itself. Its purpose appears to be 1. to prove it is possible despite the naysayers ("only large tech companies can build a browser", "no-one can build an OS from scratch") and 2. to enjoy the coding itself.
As other commenters have already stated, the only issue will be taking as much from Ladybird over to SerenityOS as possible.