There are also Merkle ladders.

What is the difference between a Merkle Tree Certificate and a Merkle Ladder?

Is this correct?:

Without Merkle Tree Certificates, the per keypress overhead for e.g. jupyter_server would be something like 3.3 KB due to the PQ signatures.

Merkle Tree Certificates basically uses the same structure as Certificate Transparency today. Merkle Ladder uses a weird variation claimed to be useful to DNSSEC. I think it's rather just to seem novel ( https://datatracker.ietf.org/ipr/search/?submit=draft&id=dra... )