Certificate transparency only uses a blockchain if you redefine a blockchain to mean "any use of a merkle tree", which is all certificate transparency logs are.
What a blockchain normally means is something that has added some sort of attempt at solving the two generals problem (https://en.wikipedia.org/wiki/Two_Generals%27_Problem), as the original bitcoin whitepaper outlined.
That is to say, blockchains have a consensus mechanism to resolve multiple writers, with some of those being potentially untrusted.
If Certificate Transparency is a blockchain, than so are the zfs filesystem (which also uses a merkle tree internally), git, and dynamodb.
Is it even a blockchain in the cryptocurrency sense of the word? It's a chain of trust where every addition cryptographically signs the previous entry (to prevent the database from being modified retroactively) but as far as I know there's no "mining" process nor decentralization - the log is still controlled by a central entity.
Not sure if the parent author meant a specific company or was just referring to the general domain of certificate verification. There are certificate verification projects in the blockchain space that go beyond what you describe.
The parent post capitalized Certificate Transparency, so they probably meant the Certificate Transparency project, as described here: https://certificate.transparency.dev/
I hadn't seen any of these blockchain projects that try to provide a similar audit log for issued certificates that you reference... mind naming them? Particularly any that can actually be used?
