PO is not part of the Metal Gear saga according to Kojima.
"The games in the series that I personally designed and produced. Metal Gear on the MSX, Metal Gear 2, MGS1,2,3,4 and Peace Walker and now Metal Gear Solid V are what constitute a single Metal Gear saga. With Metal Gear Solid V I'm finally closing the loop on that saga. In that sense this will be the final Metal Gear Solid. Even if the Metal Gear franchise continues, to me this is the last Metal Gear. "
Source
Also PO has never been part of any official timeline. Before the whole drama with Kojima and Konami, Koji Pro had an official timeline on the Konami website which didnt include PO There is now also a new timeline on the Konami website, without PO as well. And on the promotional boxes of TPP the official timeline again excludes PO.
And in a more recent article by Japanese press who attented the MGSV bootcamp, they discussed the series' canon (focusing on the Big Boss story) and they excluded PO. The info on the canon was given by Koji Pro during the event.