If both operators and are linear with amplification factors indicating stability
The adding of the operators requires a re-analysis of the combined scheme. It might be stable or not, even if Eq. (219) applies.
The individual analysis of a non-linear hydrodynamics and a complex non-local radiation transport scheme is difficult enough. It is convenient to have a combination method, that - at least in some simple cases - guarantees the stability of the combination
Here, operator splitting is superior.