Actual paper here is more understandable than this article - https://www.ndss-symposium.org/ndss-paper/airsnitch-demystifying-and-breaking-client-isolation-in-wi-fi-networks/

First, Wi-Fi keys that protect broadcast frames are improperly managed and can be abused to bypass client isolation. Second, isolation is often only enforced at the MAC or IP layer, but not both. Third, weak synchronization of a client's identity across the network stack allows one to bypass Wi-Fi client isolation at the network layer instead, enabling the interception of uplink and downlink traffic of other clients as well as internal backend devices.