What about those who don't overclock?
The 1400 will still be faster by a big margin.
HT alone will give a rather big uplift, but the cpu's you are comparing also have a big difference in frequency:
the i5 - 6400 has a boost frequency of 3.3GHz, the 1400 -> 3.45GHz. (probably gives similar ST performance)
The base frequency of the 6400 is 2.7GHz, the base frequency of 1400 is 3.2GHz.
So in any application (including games), the difference can go up to:
+ 4 additional threads
+ up to 20% frequency difference!!
tbh, the i5 6400 isn't competing at all;
They either need an i5 clocking well against the 4Ghz (to offset the lower amount of threads. note: the difference between 4-> 8 threads is higher than 8->16threads)
Or they need to enable HT on the i5 range to compete.