The FreeRTOS itself does not have an integrated TCP/IP stack. There is an official stack available, but it's only free if used on certain hardware. If you use an MCU that's supported by the OS but not the stack, then you'll have to pony up or provide your own TCP/IP stack.
TL;DR: the flaw is not IN the FreeRTOS. It is beside the FreeRTOS.
Case in point, the bugs in this article do not touch my current employer's products. For while we do use FreeRTOS, we use a different TCP/IP stack; one not listed here. (No, I'm not going to tell you which one.)