But those developers write software just like you! These products are also "software" to some degree!
The link you've posted also applies to those who build smart lightbulbs.
> seems a lot less common
I guess the problem is that the world is bigger than you may think. I don't know what you build, but let's pretend it's some kind of web service.
Consider how many brands of microcontroller-based products there are in the world, with their many variants. For the sake of the discussion consider just the "commercial" products you can purchase on Amazon. My guess is that they surpass the quantity of web projects out there.
Add to the mix the custom/industrial devices, special tools and computers, etc. that many small companies are building right now (like the one I work for).
Remember that someone is writing code for these products. Given the quantity of product, they can't be "a lot less common".
To answer your question, and if you do web services, let's say you want to know how Redis works internally. You need C.