I'm British and that is what we do. But that is the label applied to each floor in the lift (some places omit the 13th floor label) and some have floors below

I'm British and that is what we do. But that is the label applied to each floor in the lift (some places omit the 13th floor label) and some have floors below ground. However that doesn't change how many floors a building has. The article is written by an American residing in the US about a building in the US so using American conventions would seem appropriate.