The GIL does not protect that code anymore than it does this equivalent Python code:

  if len(lst) >= 3:
    lst[0] = "..."
    lst[1] = "..."
    lst[2] = "..."

Neither is thread safe with respect to ensuring that multiple threads don't modify lst in an interleaved fashion. If that's a critical section, then it has to be protected with a lock other than the GIL.