This seems like an odd error for the article to make. The great pyramid in Giza is nearly 50% taller, was built nearly 2500 years earlier, and still stands today.
At the upper bound, it was (almost) taller than the great pyramid (146m originally).
Words are fuzzy concepts meant to convey ideas. Etymologically, I imagine the pyramids are "buildings" in that they're things that are built, like you're describing, but in my internal idea of what a building is, pyramids are too close to just being a carefully shaped mountain. There's also the issue of the fact that they weren't designed for _any_ living person to stay or visit, pushing it closer to a "monument" or "memorial" or something than a "building" in my mind.
Interestingly, if there were more internal space or if they were actively used by people (at any point in history) instead of just being left to the dead in the sands of time then it'd be a lot less clear-cut for me.
I'm not totally sure how other people use that word, but a (obviously biased) survey of the top 10 people I knew would care and respond quickly unanimously called the pyramids of giza "not buildings," the most common alternative description being a "monument."
Some of the criteria are questionable to me. For example, some monuments or memorials are still buildings, such as a mausoleum. Would your survey respondents deny a mausoleum is a building?
I could also ask myself, would I consider the Statue of Liberty a building? It is in active use; it has some usable interior volume; it is free-standing above the surface. And yet I hesitate to call it a building more than I hesitate to agree a lighthouse is a building.