To do that, you'd need to disassemble the phone to insert your implant. That might be hard to do in the field (ie. not in a repair shop/lab setting with plenty of tools lying around). Not to mention the difficulties of designing and manufacturing an implant. How are you going to get it to fit? I don't think there's a lot of empty space inside a phone. How many variants would you need to design and carry around? I'd imagine that the iPhone SE would need a different implant than the iPhone XS, for example.
A bootrom attack allows you to replace all of that with plugging in your victim's device into your "hackbox" for 10 seconds. Vastly simpler to execute for your typical goon/henchmen and way less likely to get detected.
Agreed there is a substantial difference in difficulty between the attacks. I am only speaking to the parent's point about the phone somehow previously being secure and now not being secure. The only thing that's changed is the difficulty of the attack.
There are easier physical attacks too: for example just replace the whole device with an identical one you control. Replicate the target's lock screen in software and capture their inputs.
