Despite the efforts of some prosecutors, nude photographs are not inherently child porn.
Photographs must be sexualized.
What does sexually explicit conduct mean? There is not and can’t be a single all-encompassing definition. That doesn’t mean that all nude photographs are or even that all nude photographs in Penthouse are.
AFAIK, no one has ever been prosecuted for owning that Penthouse, so the assumption that the photos are child porn is probably wrong.