From the article and their own words then technically Buttigieg isn't a first out Cabinet member since he hasn't been confirmed by the Senate. The worst part is that even with the quoted piece below, it doesn't dispute the fact that Grenell was still "first out Cabinet member"

While President Donald Trump appointed Richard Grenell as acting Director of National Intelligence, a Cabinet-level position, Grenell wasn’t confirmed by the Senate and only served as a seat warmer for the incoming DNI.