Ra?
Unknown, but they are obviously far, far beyond the capabilities of anybody in recorded history.
There are two problems with that that bug me though.
One, obviously they didn't just spontaneously appear, somebody made them. And they appear to be custom made for this world with its connection to the summoned path, so they probably didn't get imported from another more advanced universe somehow.
Two, while it's a common trope in fantasy (and sometimes even science fiction) for there to be wizened ancients with knowledge far beyond modern humanity, technology doesn't actually work that way. It starts out terrible and gets more and more sophisticated over time as people improve on it incrementally. There had to have been some well established tradition of sealmasters working on R&D for centuries to advance the art enough to make something way beyond what modern sealmasters can accomplish. And that's the sort of thing that's hard to hide. That sort of sealtech would have vast societal impacts.
So here are two hypotheses to explain the summon contracts' existence.
One, a highly advanced sealing tradition did exist at some point in the past and some sort of massive sealing failure wiped the knowledge out of existence, along with all the advanced seals and whatever technology relied on them, leaving only the summon contracts. Perhaps because the contracts bridge two worlds their design rendered them resistant to a cataclysm that only affected one of those worlds.
Two, the summon contracts come from a highly advanced sealing tradition that created them in the future, and then deliberately sent them back in time because sealing was invented by figuring out how the summon contracts work and the future seal masters realized what had happened and that they had to close the loop to preserve history and their own existence.
Incidentally, this would neatly explain any invulnerability; the contracts need to continue to exist so they can be reverse engineered and recreated to be sent back. Their future existence has already happened so it can't be undone.
Maybe one of the future seal masters is future Hazou, if he figures out how the summon contracts work and how to make more and how to manipulate time besides. You never know.
Can anyone think of any other explanations?