That is simply not true.
Once you check in, you're checked in. If you show up with 5 and have a reservation for 4, you've checked in so they will NOT charge you.
I've even checked in and told them I wouldn't be able to dine because I wasn't feeling well. I was not charged. The woman checking me in said since she had me down as checking in, I wouldn't be charged.
And if you have a reservation for 4 and only 2 show up, you will not be charged for the 2 missing people.
I disagree with not telling posters all the possibilities that could happen. Disney has rules and guidelines and it's up to CMs, sometimes a manager if they wish to override the system. At any given moment a different result can happen. Technically this is true and can happen. Just because any one of us were given a pass does not mean it will happen for the.
I agree that once you are checked in that you should be good BUT if you show up with more than your ADR and they tell you that they can not seat you and you choose to walk away - you are not checked in. Just like I can't check in until all my party has arrived. Just like folks have stated that they arrived late, missed their window but the CM allowed them to wait for next table - they still got charged the no show fee because they technically did not show for their ADR. These can be reversed when Disney can see you were seated but the system is there to charge you if you are not checked in to be seated.
When you showed up and said you were ill, the CM made the decision to override the system.
Disney has never charged for persons missing from a party as long as some showed up.
Having a credit card guarantee was not to help all those trying to book against double bookers it was so Disney didn't have empty tables waiting on no shows who went to their other reservation. They need to seat close to capacity and sell those seats to maximize profit. It's a puzzle with many factors to get the room full yet not over capacity. Squeezing people into a table is not the issue, they aren't concerned with your comfort - it's about fire code capacity (why an infant must be included) and the safety of the aisles for servers and guests. It's not as simple as throwing an extra chair at a table.
Every guest who is trying to show up with extra guests needs to be prepared for all scenarios and one of them is guest being refused (particularly at popular ones, they are on to the "add a friend" story - not stating that is OP but it common as Mr. Scupper says), guest decided to leave and since they can't check you in and seat you, guest being charged. And honestly they should be, they made a reservation, knowingly brought extra guests and chose to not utilize ADR when extra guest could not be seated.
DISNEY-
"Your credit card won’t be charged before you dine with us unless you either don’t show up for your reservation or cancel after 11:59 PM Eastern Time on the day before your arrival. So, we encourage you to keep your reservation and enjoy your dining experience!"
So walking away from a restaurant because they can't seat your guest(s) not on reservation is technically canceling day of.