I think there is value in making incremental progress here on the library evolution story where possible. SE-0192 has its own set of concerns which are related but different to this proposal. Right now there is value in exposing @inlinable as a tool some library authors can use, particularly those vending packages via SwiftPM, because of optimization barriers in place because of the current capabilities of the tools. Further, since we know the desired semantics of @inlinable there is value in releasing it now for people to use. Lastly, many projects won't care about versioning when using @inlinable, but versioning is inevitably tied up in the design if pulling this functionality into syntax such as @available.
With SE-0192 the main reason to make the frozen versus non-frozen changes apply to the Standard Library and overlays only is because there is little value to do otherwise for the general Swift ecosystem at this point in time. That's not the case here — which I think is a stark difference in how these two proposals should be evaluated.