No cell phone.
And seriously, one can wiggle around all they like and make whatever excuses they like, but that loosey-goosey appointment is still a promise of availability. If your equipment and circumstances allow you to still be available at the supermarket then that’s cool and all, but the promise of availability has still been made.
Of course, in situations where people ask for an RSVP there’s a lot more promised than mere availability. You promise there will be sufficient seating - can you have additional tables shipped in at the drop of a hat? Can you produce twice as much food when a person who never responded suddenly drops by with his +1? Can you arrange for an extra hotel room, an extra lift ticket, an extra bus?
Some of us keep the promises we make. When I invite somebody over, that’s a promise that they can come over - and that I’ll be in a position to entertain them if they do.