I tried to ask directly to Mr&Mrs Smith about it
I got this answer:
Thanks for getting in touch. We're very excited about the upcoming launch of our partnership with IHG. All participating Mr & Mrs Smith hotels will soon be bookable on IHG.com, for you to earn & redeem your IHG Rewards Club points as you do on the rest of the IHG portfolio. As bookings will be made through IHG, feel free to contact IHG directly for any further questions, or booking inquiries when the time comes.
...I asked directly to IHG but not been answered yet.