Standard ads work this way by default: a brand builds a campaign in Ads Manager, targets an audience, and the ad only shows to that audience. It never posts to the brand's own timeline. Dark posting is standard behavior for almost all paid social, and the term mostly comes up to distinguish a pure ad from a boosted organic post that also sits on the page.
In UGC ads deals, dark posting happens through a creator's account instead of the brand's. A creator authorizes the brand to run ads under their name, through whitelisting, partnership ads, or Spark Ads, and the brand can build ad-only creative that a viewer sees in their feed but that never appears if someone visits the creator's profile. The creator keeps full control of what actually posts to their own page.
On Socialync's job board, a listing states whether the deal includes ad access before a creator applies, and the licence length sets how long that access runs. The creator's organic feed is never touched by ads the brand runs through their account.