I would expect apples quality control processes to pick this up. They’re so closely involved in the chip design process that it’s hard to imagine Apple’s engineers are debugging wouldn’t notice something was amass.
Not to mention the technical challenge of quickly understanding and editing Apple’s designs from the limited information that is shared with the foundry.
The device could be dormant until it gets a signal, meaning Apple won't find it unless they cut up the die. And they could attach it to anything that looks like a bus, then figure out how to exploit it later.