Presumably Apple ensures there is nothing nefarious in the hardware, but it seems an unlikely avenue for compromise. Most of the "phone" is Apple-provided software.
In theory sure, you could have a chip snooping on the bus. But it would have to have a lot of OS-level knowledge and then how would it exfiltrate the data without OS-level access to the IP stack?
Like the Bloomberg/Supermicro story, I am extremely skeptical.
A Chinese-built phone that comes supplied with an OS, that's a totally different matter.