Ask.Fedora is very active, day and night. This would overburden moderation (processing flags would be no longer a case of 3 minutes), or end up in a denial of service for users. A bad thing especially for those with more immediate need for help.
That said, the mix of our plugins and our very active user base, filter most spam and such long before it becomes a problem. I have to say the system flagging real users as spam is more of a burden at the moment than actual AI or spam cases (at least for those with urgent issues), but at the moment we can guarantee that every day at least one mod, usually more than 1 mod (and usually more than once a day:), is here and clicks through the flags.
“AI attacks” are seldom, but occur. While most are tackled the same way as spam, there are some that ain’t. But those that ain’t be tackled that way have a realistic chance to pass a “first post approval” anyway, as they have become hard to identify, and sometimes we need many posts going along with even a discussion to be get the details straight to be sure if it is AI. A moderator would need to invest a lot of time to exclude these cases from new users’ posts, if possible with certainty at all.
I have to say, at least in the current context, I prefer 10 undiscovered AI accounts that make users talk rather than blocking 1 real user who needs help or who wants to contribute (us blocking others can be also construed in problematic ways → we have seen a lot…).
Imho, in our specific Discourse context, the negative impact of such a function would exceed the negative impact of the cases we had.
However, I could imagine in other Discourse instances things behave different. My experience is limited in our Discourse instances to be honest, but I think most have not such an active user base as ours: many tl4, even more tl3, and a battalion of tl2
many of them are active in flagging, and many are established in the community and have learned how things are and what triggers some ai or spam cases have. Average users have become an integral part actually (here I think ask.fedora makes a difference between our and other discourses).
The interaction of the trust level user base flagging and mods being limited to process the flags in such cases works quite well (of course mods also flag if they see something:).
Sorry that I have not the time to read through the whole thread, but wanted to add my perspective to elaborate my “why” and ensure at least one mod gives some feedback 
Such things are hard in our case. The “occurring” FAS and Discourse accounts are actually quite separate (despite a common login), which leads to some special behaviors in moderation. Elaborating the behavior between FAS and Discourse would exceed this topic, but the short answer is: we can use the login of FAS and transfer badges (which actually is transferring the badges out of FAS into the Discourse account by a script triggered by a magical topic → not “one account with its badges” from the systems’ PoW), but usually that’s it, at least at the moment 
Given your interest in how moderation works here, another interesting hint: when we block a spam user, we have to delete their Discourse account, and then disable their FAS account → two actions. Not optimal, but not the highest priority at the moment, as we have established processes that work well to mitigate that. Our new guidelines that hopefully are to be published soon will elaborate these processes in detail, reviewable by everybody