I'm not sure I would say that spacetime is an absolute, first we need to define what an "absolute" is. One can certainly define what an invariant is, but invariants are measurable outcomes, and you can't measure "spacetime". You can predict what a clock will say, and imagine very long rulers, but spacetime is always a means to an end, not an end in and of itself. It is a mathematical milieu for making accurate predictions about experiments, what else can one want from it? The math has rules that allow us to identify invariants, does that make it "absolute"? Again I would not find it terribly surprising if a thousand years from now we don't use spacetime to predict experiments on the fringe of what is possible.