Apple is made of people. People who change, people who come and go. Maybe Apple today wouldn't, but tomorrow Apple will be a different company.
Apple's OS design philosophy makes this extremely difficult, even if someone wanted to sneak it in. The OS notifies of such activity (mic active) on a system level, and the permission model requires explicit permission at time of first access attempt, even for most of Apple's own apps.
How could you know this though? There's no source code to audit or anything.
My point is that they could if they wanted to. Granted, they don't need the money, but still... it's something you can easily hide and nobody would be able to tell without spending weeks looking at disassembly.
Oh, but they do need the money. The goal of the company is to maximize profit for the shareholders.
The "maximizing shareholder value" idea is a recent and inaccurate belief.
Corporations are beholden to their shareholders. This is an obvious and undisputed fact, rather than a belief.
The article you linked questioned the legal and ethical justification for this fact, not its accuracy.
I don't think the word "fact" means what you think it means.
You're putting way too much trust on a company that constantly screws over it's customers to earn some more money by removing ports, putting functionality behind paywall (looking at you Safari extension costs), etc. They're better than Android phones in these matters but their financial position might change some day and they might do it.