Absolute difference survival estimate rates stan_surv

I have completed a Bayesian survival analysis with the stan_surv function of rstanarm, following @sambrilleman arXiv paper (Brilleman, S. L., Elci, E. M., Novik, J. B., & Wolfe, R. (2020). Bayesian Survival Analysis Using the rstanarm R Package. arXiv preprint arXiv:2002.09633, https://arxiv.org/abs/2002.09633 )

It is a comparison between 2 groups (treatment A vs B), so I have obtained the posterior distribution of the hazard ratio and the estimated survival curves with posterior_survfit function. It is common in medical literature to report not only the relative reduction (the hazard ratio) but also the absolute reduction in survival (or incidence). I am trying to compute the absolute difference in the survival estimates between groups but I haven’t been able to retrieve the survival estimates

The emmeans package is helpful for this but does not support stan_surv models.

Any idea on how to obtain the survival estimates and the absolute survival difference?

Any help would be appreciated. Thanks!