I don't get it. This would not increase trust since we can not check if the source actually corresponds to the binary in the processor. It would need to be repeatably build-able with the result matching the signature of the default blob.

They could release a signed NO-OP firmware that is so small it would be easy to verify as benign using a disassembler. Potential of more secret code is always there, but being this malicious would be highly damaging to the company.