The noon check-in is really just a chance for a hotel to give something they would normally just waste (i.e. having a needlessly empty room just to "spite" you that it's not 1pm or 3pm or whenever their official check-in time has been set). Not a tangible benefit - especially if it's only "if available" which means they aren't required to give it to you unless they choose to.

Note that the 4pm checkout is also "if available".
I have been denied it (actually with Cent it's a 6pm checkout) in the past.

I have NEVER been denied an early check-in (without using a cent or plat or any other elite card) as long as I asked for it politely and if the hotel was not fully booked the night before (and guests hadn't checked out before).
I have even received a room key as early as 9 or 10am on more than one occasion, in various hotels and countries. = Ask nicely, and if the room is available you will likely get 12noon check-in: Platinum or no Platinum...