I got a bit confusd, after reading the battle Oro vs 3rd

Oro uses Edo, to summon 3 coffines:
1. 1st hokage
2. 2nd hokage
3. got ownd by shuriken

3rd hokage uses the sealing method that sealed the kyubi in exchange for the life of the 4th hokage.

From what what was writen, the souls taken / sealed with this jutsu are to fight for ever inside or something. Also, from what i understand, you CANT summon someone who was/used this jutsu ( look fo topics bout kabutomaru and edo - most ppl on the www say that the 1 / 2 / 3 / 4 hokage cant be summoned due to edo. )

Now for the question, if the 4rd cant be summoned by kabutomaru with edo, the who was in the 3rd coffin in the fight oro vs 3rd hokage ?

Am i wrong bout something or is it just a flaw from the author of naaruto ?