The guests immediately before and after you did not have ANY trouble with the hot water - both plan to return and one even said so in their review (review online).
The day you checked out, the independent housekeeping company cleaned the home and ran the hot water, all 3 taps wide open, and clocked "steaming hot water" for just under 18 minutes before it began to cool. I stopped by and found both burners in the relatively new hot water tank were working fine (my concern the evening before was that one of the burners might have malfunctioned).
As you know, after you contacted Airbnb to complain about the lack of hot water, AIRBNB immediately informed me, that if I could NOT fix the hot water that evening YOU would be checking out that night, receiving a full refund for the night.
While I had no objection to your receiving a full refund if you checked out, I did object to your: 1) not allowing me to stop by to try to fix the hot water heater for you - you literally put me off from 5:43pm - 9pm that night and I was only 20 minutes away; 2) that while communicating with me, you were separately pressuring my reservationist to authorize a full refund for the night, and as an Airbnb host yourself, had advised my reservationist to take your advice to process the refund and allow you to stay for free; and 3) that when you learned my reservationist was not able to gift the free night to you before my having an opportunity to check on, and if necessary fix, the hot water heater, you unexpectedly informed me, just before 9pm, that the hot water had returned, all was well and you wanted to spend the night after all, so you could be up at 4am to catch your 5:30am flight home. You kindly accepted my offer of $75 for the inconvenience of your having had to heat water on the stove to warm your baby's evening bottle.
I've been a vacation rental host for over 10 years and something like this occurred once before. It is disappointing to experience, but part of the business, and as a fellow Airbnb host yourself, you will know that such behavior is rare. But what disturbs me the most in this instance, is that a fellow Airbnb host would behave in the manner in which you have.