The latest generation of satellites use software-driven phase array beam forming antennas, effectively behaving like it has hundreds or thousands of tiny spot beams. That allows it "talk" on the same frequency to different receivers much closer (geographically) to each other than traditional systems.
Think beam-steering MIMO WiFi where a base station can direct the vast majority of its energy directly toward a client, meaning clients in other directions never see the transmission and so the noise floor is lower and bandwidth higher. Similarly such systems can "see" which direction a transmission comes from and so distinguish multiple clients transmitting simultaneously - much the same way your eyes have no trouble seeing two people waving at you from different directions even if they're twins and dressed identically: the spatial information is sufficient to tell them apart.
To a rough approximation the delta between the theoretical limit and practical limit is computationally driven these days and we've gotten really good at the computation part.
Think beam-steering MIMO WiFi where a base station can direct the vast majority of its energy directly toward a client, meaning clients in other directions never see the transmission and so the noise floor is lower and bandwidth higher. Similarly such systems can "see" which direction a transmission comes from and so distinguish multiple clients transmitting simultaneously - much the same way your eyes have no trouble seeing two people waving at you from different directions even if they're twins and dressed identically: the spatial information is sufficient to tell them apart.
To a rough approximation the delta between the theoretical limit and practical limit is computationally driven these days and we've gotten really good at the computation part.