Yeah but Milfy City is a complete sandbox game, this game has limited-roam now and then but that's probably 5% of the game content and most of that "free roam" it's to unlock rewards. And limited because mostly it a house or a few rooms.A visual novel is one slide of image after another image with texts, where it can have branches with choices.
Take Milfy City for example, it doesn't have VN tag, because MC can move around the city during the day.
MC is not forced to jumped from last image to the next image which forms a sequence of story.
Freeroam is not the same as Sandbox, Visual Novels can have a free roam system, in this case, that system is part of the visual novel (as a game). In this game, you can go back and forth in the room/rooms when the freeroam is available until you choose to end it. But it's a system within the Visual novel.