I dont get how that explains it ......
Okay i didn't think i would have to go so in depth to explain this as it's a very simple concept. So here goes.
Go back and read this,
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Tayuya summoned 3 monsters and shikamaru takes control of them with shadow possesion. Because they're about to attack her she Release them, thus unsummons them.
Release and Seal are 2 different things, infact the words are exact opposites. To release you let go, To seal you confine.
The hand signs Itachi obtained from Kabuto were the same signs used to summon the Edo's. The Edo's are all in scrolls and are a contract of a form, hence why they're summonings. So to Unsummon or "Release" the same method is applied.
Release does not mean release them from their souls. But in the case used in Edo since the people are back to the Pure world and not in the living they are no longer binded and are "Released". Do you get it?