I think your premise is a bit flawed:
humans shouldn't be expected to be deeply familiarized with the intricacies of what makes a post more suitable for...
I am a human (I think?) and I consider myself quite sufficiently familiarized with the scopes of those sites. Certainly well enough to know, given a question, which one it would be a best fit for. I probably wouldn't feel comfortable, in some cases, telling other people which site is best for edge-case questions, but I'd be happy to direct them to the right people.
In fact, I'd argue that those very intricacies are why a computer will not be able to take a question and categorize it into one of those for a very, very long time. That would be an awfully complex algorithm meant to circumvent what ultimately amounts to reading eight /help/on-topic pages, once.
I recognize that you're looking at the bigger, one-hundred-thirty-some-site network, but looking specifically at your given examples, which may or may not be more challenging ("I have a debugging problem, should I ask it on Beer Stack Exchange?" Probably only if it's a really difficult debugging problem.) But let's just look at those examples, and read their elevator pitches, available at /tour.
Stack Overflow: Stack Overflow is a question and answer site for professional and enthusiast programmers.
Ask Ubuntu: Ask Ubuntu is a question and answer site for Ubuntu users and developers.
Web Applications: Web Applications is a question and answer site for power users of web applications.
Server Fault: Server Fault is a question and answer site for managing information technology systems in a business environment.
Unix and Linux: Unix & Linux Stack Exchange is a question and answer site for users of Linux, FreeBSD and other Un*x-like operating systems.
Webmasters: Webmasters is a question and answer site for professional and enthusiast webmasters.
Super User: Super User is a question and answer site for computer enthusiasts and power users
Programmers: Programmers Stack Exchange is a question and answer site for professionals and students in software development and related fields who are interested in getting expert answers on conceptual questions about software development.
Note, these are just the elevator pitch. This isn't a guarantee of scope, nor is it a particularly detailed introduction to the site. But it's quick and easy, and it illustrates my point nicely. If I wanted to ask a question, I could look at the sentences I've quoted right here in this post, and probably 99% of the time, I'd pick the right site.
Programmers and Stack Overflow might be a bit confusing, but because of that, there's tons and tons of materials out there devoted to telling you which site to ask on.
And in any event, this tool you're suggesting, to handle the edge-cases, already exists! Per-site metas are an excellent source of information on whether a given question would be a good fit for that main site. In broader cases, where you're not even sure where to start, MSE (this site, here) is a great match. The issue isn't a lack of tooling.
I think, honestly, that the issue is that people expect this to be hard, because they hear other people talking about how it's hard. Think of the 140 sites as people (we might as well, they have community personalities and, call this a stretch, "interests"). That's smaller than most high school classes.
Yeah, there are definitely edge-cases, and I'm not saying I've always known 100% which site a certain question belonged on, but there are ways, good ways, to find out, and there are people who are more than happy to help individually to answer questions about scope.
If meta sites become overwhelmed with non-duplicate, well-formed, reasonable questions about "would this question be on-topic here?" to the point that the users just can't keep up, then I think it will be time to work on extra tooling to help users understand the scope. Until then, you're circumventing a good solution to overcome a problem that doesn't exist yet.
Now, that all said, I feel like I need to add another comment on the "aren't these subsites just another type of tag after all?" question. This one's a bit briefer. No. Just, no. No they are not. Nope. That's not it.
More on that here: Cross-visibility between SE sites; confusion of appropriate SE for a question; multi-site UI