Bitcoin.org is the site from the Core team and the official Bitcoin site. They allow you to download their software, obviously. The alternative full nodes that are available are irrelevant to Bitcoin.org and the Bitcoin Unlimited site doesn't offer for you to download Bitcoin Core.
Technically, the Bitcoin Core site says that although it shares hosting with bitcoin.org, it's maintained separately:
About the Bitcoin Core pages on Bitcoin.org
Bitcoin.org hosts several pages about Bitcoin Core as well as the Bitcoin Core downloads, but the Bitcoin.org and Bitcoin Core open source projects are run by separate teams.
So I could understand the Bitcoin Core pages in particular not linking to downloads for other clients, but would have expected the main bitcoin.org pages to be a little more open, transparent and impartial.
Also, if we are to maintain this notion that bitcoin has no owner or controlling central authority, we need to dispel this other, entirely contradictory, notion of an "official Bitcoin site".