The short answer is that it's safe because beyond that point the system can handle an interrupt without leaving its critical data structures in an inconsistent state.
More specifically, after that point ThreadX's internal data structures are in a consistent state such that an ISR can invoke permitted ThreadX services safely - whereas before that point they are not, and so ISRs must be prevented from running.
Note that the M4's exception system - especially the SVCall, PendSV and interrupt exceptions and their relative priorities - facilitates implementation of a preemptive multitasking system without requiring explicit disabling and enabling of interrupts at all.