Also known as passing the buck
"They [Qualcomm] stated that there was no plan to add signature verification for these chips. However, Microsoft responded that it was up to the device vendor to verify firmware that is loaded into the device."
Fortunately for M$, it's really on the device manufacturer. Firmware updates really have to be signature-checked, or in the least disallowed, unless (for instance) a "dev mode" pin is connected ... and even that should burn an eFuse to void warranty (Samsung KNOX Warranty Void style).
There's really no semantic working-around this incompetence.