That sounds fairly unlikely (the BIOS does executes some code from PCI devices, but he says this happens on a machine without any weird PCI device, and PCI ROMs

That sounds fairly unlikely (the BIOS does executes some code from PCI devices, but he says this happens on a machine without any weird PCI device, and PCI ROMs can be dumped too). We would not have to discuss whether this is the case if BIOS dumps turned out to be different. The issue is that 3 years after discovering the malware, he hasn't even tried to dump his BIOS. Compression and encryption at this stage would be obfuscation more than anything else, and it's the job of malware researchers to break these kind of obfuscation layer. But again, we have no proof of this even being present. The BIOS is not a black box that can't be analyzed.