1. Take an Intel or AMD system and try to secure-ify it. This is what vendors like system76[1] and Purism[2] do, especially aided by the coreboot project[3]. I'm not an expert on the details, but an important goal in this space has been disabling Intel ME[4], which I note in the original parent article seems to be part of this exploit (at least when used remotely). AMD has a corresponding parasite cpu-in-cpu, not sure about progress on disabling.
2. Use another CPU. Someone else mentioned the raptor project, that's all I know of (but again I'm not an expert). But by far the most common consumer CPU that competes with Intel and AMD, who make x86 processors, is the ARM platform used by phones, Raspberry Pis, and many embedded devices.
Note the CPU is only one kind of closed firmware that can have backdoors or insecurities....
[2] https://puri.sm/