I think $50 F&B credit is more than reasonable, it doesn't actually cost the hotel $50.
Regardless of whether a member is platinum or not if a hotel has sold time in the hotel, it is not appropriate to deliver less than that to paying customers after the sale has been made.
This isn't because of some situation that arose that was beyond their control that I would have sympathy for. They made a commercial decision to provide more services to some guests at the expense of others.
In most situations if you overstay your check out times, a fee would have to be paid. Sure as elite members we may be able to request a later checkout but that is a benefit of loyalty. For example a coffee shop might give you a free cup of coffee for every 9 you purchase. That does not make it alright for them to give you slightly less than the standard cup of coffee one day because they decided to provide others with more and didn't have a sufficient amount left.
In most situations hotels would rely on not all guests arriving at exactly 4pm to cater for late check outs. In this case they obviously realised that with the number of late check outs occurring they would likely have a situation where they could not have sufficient rooms ready for guests checking in. I think it would have been much better for them to offer a free drink and maybe a snack for anybody who arrives at the hotel and could not check in by the advertised time. This should have been stated in the email.
On occasion where this has happened to me, they have always been very apologetic and offered a drink or snack at the bar while waiting. It doesn't cost them much at all, keeps the guests happy and the least they could do for not meeting what customers have paid for.