A mini MGS would be like the Tanker Chapter in MGS2. In the Tanker chapter you collect and use different gadgets, have a boss battle, have a unique action senerio, have a unique stealth section. For being a short chapter, the Tanker chapter takes you through many diffrent things. You have your regular stealth that mixes it up with diffrent patterns, the action scene in the hallway with the guards, the Olga battle, and the stealth section while taking pictures. None of it feels like filler, it all feels like it's in service to the story thats going on. Plus, you do get plenty of story.
Ground Zeros dosen't have a boss battle, a unique action scene, an interesting stealth section, or a lot of story. You go from point a to b and it takes little more then an hour if your slow. Even the presentation is lacking when it just decides to tell the story through text or not even have that assault on mother base be playable. Hell, they couldn't even give you a new trailer for MGS5.
The rest of the content is thrown together missions that no one would want or care about in an actual MGS(well maybe if you liked that shit in PW). It may be fun merely because the gameplay is fun, but it dosen't have that hand crafted feel that an actual MGS has. Just feels like filler crap.
It is a demo, because if it's a mini-MGS then it's pretty bad. Even Kojima described it as a game made so people can get used to the way MGS5 plays. It's a slice of the game(even if it dosen't appear in MGS5) made to get you acquainted with MGS5. Oh and make some money while they make 5. Sounds pretty much like a demo to me.