At another point though, it was someone who did zero web, but instead could do embedded C, and native mobile apps for said devices. And the other two were more variations.
The only real abstraction I could apply that made all of these fit under the same umbrella was something like "Given any product's technology stack, where the stack contains a plurality of languages and/or libraries, a 'full stack' developer is one who is not inhibited by the variation in technologies to be able to contribute meaningfully to most of the product". Or something like that.
Are others seeing this?
I think it should be someone who understands all the layers from the ethernet hardware, arp, ip, tcp, http, https, bufferbloat, dns, caching, etc. along with the various software stacks on the servers, and clients, along with their "apps"
If you don't know how to fix a DNS issue, you're not there yet. You don't appreciate all the failure modes.
I’ve met people who call themselves full stack who seem lacking in parts of the stack — cannot navigate the Linux command line, can’t write SQL, etc.
Like so many apparently descriptive labels in the software/tech industry the term “full stack” got applied to so many collections of skills at various degrees of ability to lose any useful meaning. See also “senior” and “engineer” and “architect.”
It tells you something true about them, but does it in a way that has a lot of error. That might seem bad, but it isn't.
In communication there is this idea of progressive revelation. If you were sharing the shape of the world, you start by calling it a sphere. As greater interest is shown you proceed to pointing out it isn't actually a sphere, but an ellipsoid. Still greater interest leads to revealing that it is actually an irregular ellipsoid. Only if they are interested in getting more specific do you venture into talking about the irregularities of the mountains and the canyons. The full description of the shape isn't realistically shared - you live on the Earth, but you don't know its shape and if you ever tried to articulate it a stray gust would move a sand dune while you weren't paying attention and you would still be wrong.
Someone who is saying they are full stack is like someone calling the world a sphere - it is true to a point, but needs elaboration to produce a better understanding.
Personally I don't try to spread myself too thin across disciplines. If a certain prescribed set of requirements is made out by a potential employer, I reject the employer, not the other way around. Luckily we have that choice. It wasn't the case earlier where you had to fit into neat little boxes to get employed.
[0] https://en.wikipedia.org/wiki/%C3%9Cbermensch
[1] https://en.wikipedia.org/wiki/Great_Man_theory
With that in mind, IMO the examples you outlined seem to fit the fe/be full stack paradigm.
You should feel comfortable to work across different layers of a stack, whatever that stack is.
Edit: one clear difference I noticed between full stack and non-full stack engineers is that the former are often required to navigate problems that require architectural fluency.