Vlad The Brave example is showing two windows open, each containing one tab. I'm able to drag tabs between windows in Orion, as you mentioned, but the problem arises when there is only one tab left in a window.
There doesn't seem to be a clear cut way to merge the two windows/tabs fully in this example, as the creator of this post mentions and I emphasize in the video.
I believe this video asserts the subject at hand better: How does one —drag— the last tab from the window in the foreground to the window in the background? The blue highlight that appears when successfully draging the first tab [1] to the background window, doesn't appear when trying to drag the last tab [2] (The foreground window should then disappear when the last tab merges with the background window, because zero tabs left).
