Yep sorry, mea culpa, I see you are right. Hmm not sure if t+c's rewritten or always said it this way?

I believe IHG then has to contact hotel on yor behalf anyway.

For me far easier in first instance to cut out IHG as the middleman and ask hotel direct and to thus know where things stand in just 5minutes. If hotel will not play ball, then I might follow up with IHG I guess.

The couple of instances I remember invoking the guarantee, I contacted hotel direct and got the platinum guaranteed room offer and accepted as offered rate was ok, not an absurd figure that is sometime quoted when hotels have to offer guarantee but don't want really you to say yes.
(Admitedly was for hotels I had already stayed at multiple times which may have helped)