Many years ago, some person misconfigured their squid proxy and was hitting https://jsonip.com to the point my server at the time was turning green in the gills. I started responding with 418 "You are a tea kettle" or something like that to those specific requests. The originating dev actually paid attention to their error messages and quickly resolved their config issue. Fast forward to March this year. Some fucking…
No, sorry this is 100% on you. You should have implemented throttling, and daily query limits. Once you put something on the internet for free, you can't complain then if people are using it.
But here, you are actually taking resources from the author. That's something that is not only rude, but also puts the user at risk. The author is really nice for maintaining service, he could have just stopped it, or even served malicious payloads. After all, there is no contract saying that he should be nice. Here, despite the strong language, he is trying to stay nice instead of just cutting a service that many people rely on, which he has all right to do.
Throttling and daily query limits would make the service unreliable and not so different from cutting it off completely. Keep in mind that it is not a single user abusing the service, it is many users who got redirected there by an abusive app. He could have used the "API key" that is suggested here, but it is a hurdle that will affect usability.