I don't think that's a useful way to look at it. It doesn't take into account this most important point about relativity: it does not
consider that "everything is relative". Certain things change depending on the point of view of the observer, but other things don't. We can say that things that do not change depending on the point of view of the observer are indeed "absolute".
For instance, in your example all observers will agree that the slit at B opens before the light hits B: the order of these two events is absolute. In contrast, the order of the two events "light hits A" and "slit at B opens" is not absolute.