Now, if you trust Apple, the T2 chip is perfectly secure for this use case. Apple claims this serves as a hardware disconnect baked into the silicon, in which case it is no different from a mechanical switch.
Finally, the whole argument about user control over various security features is interesting. I will say that Apple has, better than any other manufacturer, struck almost perfectly the balance between security and usability. Apple's software/hardware security does its job, does it well, and gets out of your way. Including 20 different hardware switches and 150 different security settings that mean nothing to the average user would not be in their design language.
It's all about your threat model. Perhaps yours is a bit too severe to use traditional consumer devices.
If Apple distributed free software trusting Apple wouldn't enter into the discussion. We could inspect what Apple distributed, we'd be allowed to change what we don't like about the software Apple distributed to us, and we could distribute improved versions of that software to help others. No need to buy into uninspectable code we're not allowed to change or distribute further.
And the answer to the question you didn't answer which the original poster asked -- do you need a chip to do this task -- remains "no". Purism's hardware is proof by existence. With Purism's hardware switch there's no software involved to accomplish this feature and they (as far as I know) distribute a free software distro called "PureOS" (a GNU/Linux system) which also happens to have earned an FSF-approved distro entry.
Putting this functionality into a computer chip is ridiculous and unnecessary partially because there's no clear way for ordinary non-technical users to control that, and partially because this suggests that software (as I presume the software on this new chip will be alterable by Apple) can still affect whether the mic is hot. The headline "Apple's T2 chip will prevent hackers from eavesdropping on your microphone" should probably be interpreted to mean that Apple won't be prevented from listening in.
Camera access and wireless network access should also be physically disconnectable via a simple user-controlled piece of hardware which electrically disconnecting the on-board webcam and wireless network device from the rest of the system -- a slider or rocker switch. This too is something I understand Librem laptops offer but is uncommon elsewhere.