The "video" that plays on the title screen of Mario Kart World appears to be actual gameplay. As shown in the 2nd video, you'll be able to seamlessly enter gameplay from the title screen if you select Free Roam

https://reddit.com/link/1knck2d/video/k871jfen2z0f1/player

https://reddit.com/link/1knck2d/video/esnio8en2z0f1/player

This is a reupload of my original post, I previously mentioned how the player selects Single Player to seamlessly enter gameplay. They are actually clicking the + button to access Free Roam. This is still a cool detail about the game!