Quintin first of all I am sorry for what happend at the check-in, Our goal is always make the guests stay as much as enjoyable as we can, but as you said it was the guest checking out responsible for the delay.
Anyway being the first time we have this kind of problem, and I hope the last, I want to explain what happend.
Everytime we arrange the time of the check-in and check-out with the guests to try to save their time as much as possible.
In your case, the days before of your arrival, I informed you by airbnb that, again to save your time, even if the day of your check-in another guest would have been checking out, you had not to wait the 3 p.m.(the check in time stated in the rules) but you could have dropped off your bags and taken the keys as soon as the guest would have left the apartment at 12.00 noon the time stated for the check-out. We would have cleaned the aparment while you were visiting the city and making the place ready at the 3 p.m.
You were at the apartment around 1 p.m. and the guest before you has left the place no later than 2.00 p.m.
Anyway I looking forward to hosting you again and I promise the check-in will be smooth.