(My concern was that the twitter basically contained nothing of any content: no technical details, no link to a blog post, nothing which can be checked or verified... which makes it indistinguishable from insinuation; and so I have to dismiss what you said out of hand.)
(Regarding provocative language: I don't think publicly calling someone out as a liar, in so many words, particularly on a medium like Twitter which doesn't really allow for an effective response, is particularly effective in producing a useful result...)
Anyway:
Re the manifest issue: to paraphrase, to check I'm understanding you correctly: because each manifest refers to its predecessor, and not vice versa, adding any blob which looks like a manifest implicitly adds that manifest to the tree. Normally Fossil trusts authenticated users to add blobs to the tree, because they're authenticated, but ticket attachments can be added by anyone, which effectively means that you can bypass the authentication and commits can be done by anyone. Is that correct?
In which case, yeah, I agree; that's very bad. I can't spot any holes in your reasoning. It is possible to positively identify attachments by looking at their parent manifest, as each one should be pointed at by an A record, so I suppose you could disallow manifests if they're referenced like this, but my gut tells me that's going to be horribly fragile... you're right, adding a type prefix is obviously the right way to go.
If you create a manifest and check it in as a normal file, so it's referenced by an F record, is it still treated as a manifest? If not, could this machinery be extended to attachments as well?
You did bring this up on the mailing list, right?