Except it won't be spec.
I'd love to be disproven otherwise, but Intel made it CLEAR that it requires both protocols and that is the only reason why you won't see expresscard to TB adapters like you can now buy for USB or SATA. Intel explicitly said TB ports would need to be built-in.
Without this requirement, you could make an expresscard pr PCI-E to TB adapter because it's the same protocol.
Welcome to Steve's garden of stupidity.