Right, but the old clients that don't support Host Certificates will just keep using the Host Keys that they always have, and prior confirmation or TOFU or what

Right, but the old clients that don't support Host Certificates will just keep using the Host Keys that they always have, and prior confirmation or TOFU or whatnot. It can be of great benefit to the new clients at the same time, though -- because you need to put the hostkey and the certificate side-by-side on the sshd. Clients that have a CA known_hosts(5) entry for a given name (or a mask that matches the name they're connecting to) will prefer host certificates over host keys and attempt to verify them that way if the server presents one, but if that fails they'll still fall back to regular host key validation (unless you explicitly set the HostKeyAlgorithms directive to eliminate non-certificate-based ones). It's the best of all worlds, in my opinion.