But this is very different: This is more like the original copyright holder putting something on BitTorrent, inviting people to download it freely, and them suing anybody who seeds it: By putting the data on a P2P protocol, you've kind of implicitly stated that you're okay with people seeding, or whatever the local terminology for the same is.

Um, no.

You have no guarantees that the content put up via IPFS is by the original copyright owner.

Thus immediately after you receive the infringing content over IPFS and start seeding it, you are infringing on the copyright owners rights. You have no recourse and no legal defenses to protect you.

Well, then, as soon as you receive notification, you can immediately stop hosting the data: it's like a DMCA takedown, but you have to send it to way more people.

Too late. Once you've retrieved content via IPFS, you are already infringing. You can still be sued, waiting for a DMCA notice won't save you.