The question then becomes whether Pao didn't know about it because she wasn't paying attention to her business, or because Ohanian and/or other parties deliberately hid the information from her until after the proverbial stuff had hit the fan. I'd consider the latter a fireable offense, but YMMV.
From what's been said publicly by principals in the story, we know that Ohanian had taken the AMA section into his personal portfolio. By the way: that by itself is weird: the chairman doesn't usually have an operational role in the company, among other reasons because it's a conflict of interest. Regardless of who he was, he had ownership of the department Victoria Taylor worked in. In most well-managed companies, that makes hire/fire his prerogative, and coordination of personnel changes his responsibility.
When people screw hire/fire up and cause disruption, the CEO normally fires or demotes them, and gets extra careful about future promotions and delegations to keep there from being a pattern of poor decisions about delegation. Here, neither thing was allowed to happen: Ohanian is effectively unfireable, and Pao wasn't given a chance to not delegate more stuff to him.
I'm sure that she doesn't have to approve every dismissal, but there's a pretty big difference between Apple and Reddit. Last I heard Reddit only has a few dozen employees. I'd expect the CEO to know most of them by name.
Apple has what, 45,000+ employees? Reddit has around 70. (Assuming https://www.reddit.com/about/team/ is accurate, at least.) So I'm not sure how the the one is even remotely comparable to the other.
Moreover, my point was about key personnel, not all personnel. I don't expect Tim Cook to know or care if J. Random Genius at the Genius Bar in Tucson is about to get canned. I do expect him to know if, say, Jony Ive is about to get canned.
In other words, a CEO should at least have a cursory understanding of the status of anyone in the business who could throw it completely off track if they were hit by a bus. Given how much of its future Reddit has pinned on AMAs, and how completely non-functional the AMAs became when Taylor was fired, she would seem to have been a member of the bus brigade. Which to me at least would imply that (1) Ohanian should have known that firing her was a risky move, and (2) he should have informed Pao of that before pulling the trigger if she didn't understand it already.
Of course, if the management structure is already divided up into little fiefdoms that the CEO has no effective control over, none of this matters much.
What you are now suggesting is that, having had AMAs delegated to her boss, the charismatic founder of the company, Pao should have micromanaged the operation of AMAs and prevented Ohanian from terminating Taylor.
This strains credibility.
I think we all think Taylor was a "key employee" because of the drama that happened after she was let go, but that very few of us would, in Pao's shoes, have known Taylor was so operationally critical. Even if we had, I think all of us would have found it difficult to override a decision Ohanian personally made about a division of Reddit he was personally managing.