This opens the door for them to compete with a lot of these platforms. Since they already have the hardware on site (iPad) they start to become a competitor for StyleSeat, MyTime, Noshlist, NoWait, etc.
Yes the greatest advantage SquareUp has is that they already have a list of clients that they can advertise this to. Clients that might be using their SquareUp credit card processing system.
Appointment reminder doesnt seem much like a competition as they do more than just reminders. They also setup a site and make it easy for people to select a time that is free.
Square Appointments does those things as well according to the link. In any case, I don't understand how doing MORE would cause Appointment Reminder to cease being competition for Square.