Code change:
We don't ever really say that QUIC depends on TLS providing the properties that we claim TLS provides. So it's worth pointing that exposure. This is more so because some of the properties we claim are dependent on these properties.
(Cleaned up duplicate instances of the same comment.)
As per comment URL I'll note that there is MITM protection in QUIC, but it is not immediate which is roughly why we have the concept of handshake confirmed.
The options for triage are close it, mark as editorial, or mark as design. This has a needs-discussion, and I'm seeing a lack of any discussion. So you might see where my thought process is going... NAME this relates to your PR on the QUIC threat model. Are you willing to provide some comment that helps us triage the issue?
I think that we can take this as editorial, as it merely requires that we capture already agreed principles (we don't protect against an adversary who can inject packets). NAME assistance would be useful, but I can produce text if nothing is forthcoming.
I can always provide comments, no guarantees on how much they'll help triage IIRC, this is an issue filed from a comment on the threat model PR that was several-months merged to potentially increase the scope of what that covered. I think that the original threat model that NAME and I put together, covering the handshake and migration respectively, is intended to be the start of that section -- as additional items are discovered that could use coverage there to help clarify QUIC's stance on whatever issue, they should totally be added. I agree with NAME that adding such items seem editorial, as they aren't introducing any new requirements and are only capturing current realities as specified elsewhere. (Of course, as that text is written, it seems logical that putting it all in words next to each other may highlight issues, and if the WG decides that we don't like how the current spec results in a particular security stance, we can open a design issue to change normative text elsewhere.) So, I'd probably consider this editorial, and ask NAME to potentially contribute some text or clarify what he's looking to have covered by the original comment -- once that's clear, happy to help get a PR up if needed (or NAME text is always good too!).
Since this concerns the handshake, I'm not too confident in contributing text but I'd be happy to review and discuss. I believe it is correct that the issue is covered in existing text elsewhere but as NAME says, it makes sense to cover it in one place. The risk is that the reader is not aware how important it is to reach a certain point in the connection before trusting data, and especially to those seeking to create derived versions. From various discussions I'm not sure the importance is broadly understood. Additionally, I'm not entirely convinced that the current text fully captures the problem - not that we necessarily can - for example various stateless transmissions / redirects / token issues - might assume a level of privacy that doesn't exist in all cases, thus requiring stronger integrity elsewhere such as in CIDs, tokens etc..
Thanks folks. It seems we have a path forward, so I'm removing the needs-discussion label. The editorial process will naturally generate some back and forth but that is the same for all editorial issues.

Related discussion:
text: 21.12.1. The QUIC handshake incorporates the TLS 1.3 handshake and enjoys the <del> cryptographic properties described in Appendix E.1 of TLS13. </del> <ins> cryptographic properties described in Appendix E.1 of TLS13.  Many of the security properties of QUIC depend on the TLS handshake providing these properties.  Any attack on the TLS handshake could affect QUIC. </ins> <del> In addition to those properties, the handshake is intended to provide some defense against DoS attacks on the handshake, as described below. </del> <ins> Any attack on the TLS handshake that compromises the secrecy or uniqueness of session keys affects other security guarantees provided by QUIC that depends on these keys.  For instance, migration (migration) depends on the efficacy of confidentiality protections, both for the negotiation of keys using the TLS handshake and for QUIC packet protection, to avoid linkability across network paths. An attack on the integrity of the TLS handshake might allow an attacker to affect the selection of application protocol or QUIC version. In addition to the properties provided by TLS, the QUIC handshake provides some defense against DoS attacks on the handshake. </ins> 21.12.1.1.