It may or may not be honoured. Nobody really knows but it's clearly a glitch, not something that people will come across that often.

Like I said, AMB weekend reservations are supposed to show the actual rate for both nights. The hotel simply removes the charge for the second night during your stay. And if somebody at the hotel notices that you aren't supposed to be getting a Monday night for free, then an IHG email is irrelevant. Especially if the hotel's internal systems show something else.