Perhaps a more intuituve term than “sharing” would be “arbitrating.” An operating system allows multiple threads of execution controlled access to a ressource.
FreeRTOS’s kernel only arbitrates the CPU. In essence it’s a scheduler and a provider of synchronization objects that allow multiple threads of execution (tasks and ISRs) controlled and arbitrated access to resources like peripherals, but unlike operating systems such as Linux, there are no arbitration strategies for resources other than the CPU predefined by FreeRTOS.