I mostly focused on the DOM spec, since that is what this was about. I read that it is part of the HTML5 spec, but that can't be where it originates from, since it existed before HTML5.
a rose by any other name would still smell so sweet. who cares where it came from, it's just there. as my mom used to say when she didn't understand something: "that's just how god made it"
I guess after a year of working for an international company as a customer with countless specs and documents to write and update every day, I sort of got used to thinking "in specs". And seeing how much implementations differ in this area, it feels worth knowing to me where particular things are even specified. And there are quite a few cases where the spec matters – for example when it comes to decide whether something is a bug or intended behavior.
In any case, at least being part of HTML5 strongly suggests that alert is indeed not a DOM method.