I mean, scale is also a valid range to include, but I was more thinking about utility and stability. Like, I personally think that a single Raspberry Pi in the corner can be a home lab so long as you're learning something from it. But the thing you're learning on it can be "how do I write a toy HTTP server and the only traffic it gets is me testing it", or "this DNS server is in the critical path for every device on my network". High stakes, low stakes, big, small, careful, YOLO; if you experiment and learn things, it's a (home) lab.
To go back to your car analogy, isn't an RPi like a 25 year old econo beater? I always took homelab to refer to a home datacenter; I always assumed (possibly incorrectly) that the origin of the name was due to the analogy of someone who sets up something like a chemistry lab in their garage.
I think of it as the computational equivalent to a home machine shop. If you don't have some heavy duty equipment can you really claim you have your own shop setup?
If someone works on and tweaks and gets to know their 25 year old econo beater, I don't think anyone has the right to say they aren't a car enthusiast. It's less about the horsepower (or ghz) and more about the passion put into it. For example you could add an ad-blocking DNS server on your devices and it would work just fine - but if you put in the effort to self-host it, even on a Pi, it's (IMO) a home lab.
