You made me watch a 80 minute video...
And there's a blockchain. Not one, but more than one.
There's 5 categories of DID:
Ledger based DID: the original DID
Ledger middleware (Layer-2) DID: uses a storage layer over the DID blockchain. It should work like the Layer-2 protocols on Ethereum blockchain.
Peer DIDs: it's like a permissioned blockchain. A couple peers band together and they trust each other and nobody else.
Static DID: can be created (they are just a signed file) but they cannot ever change, nor be deactivated.
Alternative DID: everything else.
So you can create a DID for yourself, store it in one of those blockchains, and be happy. The DID never change even when your keys expire and you issue new ones, because you can update the DID with the new key: you sign the update with the current key while creating the new DID. So it proves you had the original private key and the new one.
In this sense, you would be able to have an instant login on any service: you provide your DID, the service sends you a challenge, you sign that challenge, the service checks the signature and have immediate access to any information you set as public on your DID: mail, name, country, things like that.
Now to your question.
There's the seller blockchain (or product, or marketplace, or the Universal Federated Chamber of Commerce Blockchain). This one can be a ledger based, using one of the myriad existing blockchains or a new one created just for that. The product is entered on the blockchain when it's created, and every time it changes hands, its status is updated on the blockchain.
When Merri wants to buy the card, she can check the entire history of the product since it was made, every time it was sold, and knows for sure that the card is legit.
Knows for sure here depends on how trustworthy is the process to update the blockchain, or someone can claim ownership of the Eiffel Tower, for example.
I believe the idea is to have an unique way to identify people around, in place of logins, usernames and emails. I don't see much of an improvement on the privacy area, because blockchains and privacy on the same place is a strange arrangement.
And it's kind of TLS for people. You can know for sure that a DID from Alice will have Alice's public key and wasn't tempered with.