I don’t think it’s that simple. A given version of SeaMonkey advertises compatibility with a certain version of Firefox because those two browser versions share the same core codebase. It can generally be assumed that that version of SeaMonkey can do, at minimum, everything that that version of Firefox can do. In addition to this core compatibility, a given version of SeaMonkey may also support certain features that later versions of Firefox support, though not necessarily all of them. So it would not be truthful (nor generally helpful) for SeaMonkey to explicitly advertise compatibility with those later versions.
In short, just because a new browser advertises compatibility with an old browser doesn’t mean that the new browser doesn’t support newer features as well. Again, most browsers today advertise compatibility with Mozilla/5.0 but that doesn’t mean they’re strictly limited to what that ancient engine can do.