A short explanation: most Samsung phones support rSAP out of the box. My app makes sense for those who want to use rSAP with LineageOS, which has no rSAP support. Sometimes Samsung adds bugs to firmware updates, this may also be fixed with my app. So the phone must basically already support rSAP to make my app work.
Dual SIM Samsungs usually (always?) don't support rSAP out of the box. On a few models my app works a little bit, like the S4 Duos. But even there it has massive problems, like you have to reboot the phone after an rSAP connection.
I don't understand why Samsung does not support rSAP on Dual SIM phones, there is no technical reason. MediaTek phones work fine and you could even choose which SIM card should be used for rSAP. Although this is no longer the case with Android 6+ on these phones, at least on those I checked.