Oh its therefore like the new LC40, so it comes all pre-assembled with two 18650 secretly inside? Not sure I like that... but from an engineering perspective I see why, it shortens the torch to have the threading.
The photo crudely attempts to show what and why for the new design. The older LC90 used two caps the inner screwed down on the battery and the outer cap offered the button but compare with the older LC40 which had no USB port (remove battery to recharge) and the newer LC40 contains one cap which is combined charger+button. The combining with a non-removable battery shortens the torch as you have less metal in total, reducing from two caps with sets of threads to stack over each other to one cap and presumable a smaller thread which is sealed (somehow - glue?).
If you were making the LC90 longer via two 18650 you can see in an effort to shorten they took the decision to use the same method as the LC40 of merging caps and sealing. My guess is this shaved some cost, weight, length. Given the trade-offs here I can see why and agree with what Anker have done. It is also academic as I would not buy the new LC90 anyway, the older one is just fine for my needs, and I own 3 of them.
The merging of USB port with button necessarily makes the button smaller.