The XZ backdoor was completely in the open. It only got found because an engineer at Microsoft was far too good at controlling his environment and had too much

The XZ backdoor was completely in the open. It only got found because an engineer at Microsoft was far too good at controlling his environment and had too much free time to track down a 1% performance degradation. So... no, you really cannot verify that there is no backdoor. Not against a well resourced, patient adversary. I'm not sure what your links are supposed to be proving. I'm neither of the opinion, that PCC is useless, nor am I under the misconception that a signature would provide a guarantee of non-maliciousness. All I'm saying is that, if you include Apple as an adversary in your thread model, you should not trust PCC. But not because it's closed source (or whatever) but simply because you fundamentally cannot trust the hardware and software stack that Apple completely controls all interfaces to. Personally I don't consider this a useful thread model. But people's situation does vary