This observation seems unwarranted (and a little unkind). For a voluntary personal project - not to mention one that involves a bit of personal risk/discomfort - they can publish the data in whatever format they please and revise it at will.
Usefulness of the API aside, the tone of it is full of red flags. Combined with all the other red flags, I can't help but to read it like this:
"You should be grateful I ever bothered to make an API, if it changes that is your problem. I can't bother to make this API stable or version it, this data is so valuable I assume you will make changes on your end if you hope to integrate with it."