This is literally the exact niche solution that attracted me to my current company (Electric Imp). I'm still fairly new, but generally the pitch is it's easy to get started, and easy to scale to whatever level you need to, theoretically up to millions of devices for a single product (certainly that many across the spread of customers).
I used Electric Imp in college for an electronics project -- glad to hear it's still kicking! It was a great platform, minus the custom Squirrel language (a Python or Lua library would be more familiar to most people).
Hah, that's exactly what I brought up in the interview - why squirrel?
I've talked to our CEO and the original core team about this a couple times. In their minds LUA, while 7 years ago was more widely adopted, was also less well supported and more volatile than squirrel. Squirrel got the job done, was pretty straightforward, and was small to get going. And, once the decision was made and customers were already compiling it onto live projects.... Well, at least there haven't been growing pains as a result, it still gets the job done and we have metric tons of libraries for it.
Edit: since everyone else is promoting the competitors ;) : https://electricimp.com
A shame this post didn't come up in ~2 weeks, we're about to massively update our docs, API, and frontend IDE environment.