This could actually be useful though. Even with the key you can't insert new signatures into the blockchain's past. If your key is stolen, instead of invalidating the key and thus retroactively revoking every signature you've ever made, you can instead revoke only the ones made since the key was stolen.
There are no guarantees against credentials being tampered with whenever a new version of that source is established.
Bitcoin also offers no strong identity verification.
Precisely. It becomes a federated accreditation system that's verified from consensus and identity, rather than a centralized body. It's also immutable and irrefutable evidence of achievement.
edit: Could venture to say that this model is what the future of education could look like, where transcripts are determined by your participation on the blockchain.
I feel like the blockchain only works when there is a binary yes/no of factuality, and grades can't be defined on a binary factuality.
Use blockchains for whatever you want.
It doesn't look like they actually need a blockchain for this. They just need an archive of signed documents. They could use a website, or IPFS.
Blockchains are hot right now, but they solve a very specific problem: finding consensus on a history of transactions. If you need to prove that a document was signed at a certain point in history, then a blockchain can help. But you don't need one for any of the examples given in this proposal, such as hosting student transcripts.