Regardless of it's a port or not, the game has had over 8 million players on xbox (prolly about ~7 million copies sold) and is regularly on top of Xbox's charts. By all means it's a major release.
And those are only 4 reviews. Majority of the reviews will come after 1.0 releases.
Even then I'm talking about metacritic and not opencritic, which has 0 reviews: http://www.metacritic.com/game/xbox-one/playerunknowns-battlegrounds
And I did list dark souls on switch, at the bottom. And no, niche japanese games aren't "major".
Like you said, the game sold around 7 million copies. They had physical boxes for this game at all major retailers. That means it already came out. So, no it is not a major release of 2018.
You said that nobody reviewed early access games. If there are reviews written for early access games, then doesn't that show your claim to be false?
Opencritic doesn't weight the scores with a hidden formula like Metacritic does. So IMO Opencritic is more trustworthy.
PUBG should be at the bottom too then. It's not a new release, but an update to a year old game.