Slipping on my photographer hat ...

Occasionally a public place will either contract for advertising photos or be contacted by a media company to photograph in the venue for commercial or artistic purposes. I've done it myself. An appointment is made for when the venue is closed to avoid interference by the public and a shoot is done. Works the same for movies and television.

Kenrokuen is a VERY busy place. I don't see anyone in the photos you posted which suggests some sort of special access. It is of course possible they were taken during the special event times mentioned above but those times would tend to be very popular. That lack of anyone visible suggests a private session.