Or, that looks like a single point of failure that would make a good target for the government to issue a National Security Letter to Intel for.
Right, but it can order you to turn over your private keys, like it did to Lavabit[0]. Intel might instead voluntarily hand over a signed backdoored firmware, targeted at a specific set of CPUs, rather than the keys themselves, which could do more damage if the government lost control of them.
Similarly I don't suppose it would be much harder for the NSA to get access to the physical servers that Signal is running its software on. Perhaps you could say that merely having the access credentials wouldn't be enough information for the NSA to take control of Signal's servers without them noticing, but any cloud provider that wants to do business with the government surely has special backdoors that make this easy, when presented with an NSL.