Orochimaru only had what was left to go on that enough makes it inconclusive since the village was wiped out. So that makes it likely not certain. Like I said his objective was to get back his arms.
Anyway here's the proof it's a contract which requires your soul instead of blood.
You must be registered for see links
Also only those who make the contract can see it how would the uzumaki know how to do this? We're they randomly playing with handseals and going through 10 hand seal combos and the 9 billion+ potential combinations before that to come to the conclusion these were the right hand seals.
You must be registered for see links
Here it confirms that not even Orochimaru knows of this jutsu yet the Uzumaki who were famous for sealing jutsu invent such a jutsu and Orochimaru who lived through the destruction of their home not know of it (sealing jutsu).
Now Orochimaru knew of the uzumaki clan he even had karin as his subordinate, he even knew of the chakra chains of kushina.