The vulnerability has been confirmed on Windows 10 Enterprise 64-bit (OS version 1607, OS build 14393.1198) and Microsoft Edge 38.14393.1066.0, Microsoft EdgeHTML 14.14393.
The problem is that the index in the PoC has unsigned 32-bit value of 0xffffffff, possibly because the data structure has not been properly initialized, which leads to out-of-bound access. If an attacker can put data they control at array+offset, they would be able to call this->_UpdateSelected with a controlled argument, which presumably would be sufficient to turn this into a write primitive.
00007ffd`625b3903 488b14ca        mov     rdx,qword ptr [rdx+rcx*8] ds:00000290`617a5788=????????????????