No, we do not sign anything. We do not do what cryptography understands under the terminology "signing". We also don't do "hash chains". And no, none of my objections to logtools apply to the journal, that's why I pointed them out.
And no, with the systemd journal the sealing key (that remains on the system) cannot be used to "change history". The old sealing key is forgotten and erased when a new sealing key is calculated and you cannot get back from the new one to the old one.
Next time, please read what I wrote, before commenting about the technical background of it, please.
Again: the FSS stuff is very different from what logtools does. Please read up. The journal's FSS stuff is much more useful than hash chains/signatures are. We do not require sending the topmost hash away.