[mikhailt]

That's because it is nearly the same as WebKit Nightly but with a few things included that WebKit Nightly can't include such as iCloud sync. Apple already explained on the same page as the OP link:

> You may already be familiar with the WebKit Nightly, which serves a purpose similar to that of Safari Technology Preview. For most people, we think Safari Technology Preview is a more convenient and stable way to live on recent WebKit changes. Unlike the nightlies, Safari Technology Preview supports the full set of iCloud-based Safari features, including iCloud History and iCloud Tabs. And we’ll use the time between Safari Technology Preview releases to curate and test updates to a point where we think developers will find it practical to use as their primary browser.