Well, for one thing, Project Ogre was going to be the first game to demonstrate the capabilities of the FOX Engine - in addition to being open world and featuring "taboo" elements that Kojima feared would eventually put him into trouble with Japanese censors. All three of these elements have now been incorporated into MGS V (plus, remember when reports came out of how gruesome and disturbing the ending of GZ is?).
Of course, Project Ogre was never released, so we can't make a full side-by-side comparison on all of the features from it that were transposed to MGS V. However, based on what I've seen so far, it does seem like MGS V is just a result of Kojima's failed IP that he couldn't get off from development.
Kojima never explicitly mentioned the word "reboot", but when he addressed the change of voice actors in this quote, you can certainly see that MGS V is meant to be like a recreation (or in short, "reboot") of the series:
"I can say, yes, it will be a new person. I can't say who it is yet.
What we're trying to accomplish here is recreate the Metal Gear series. It's a new type of Metal Gear game, and we want to have this reflected in the voice actor as well."
https://www.psu.com/a018927/Hideo-Kojima-explains-why-David-Hayter-is-not-in-Metal-Gear-Solid-V