Or mentionably the stance is even far beyond Stallman himself: https://stallman.org/stallman-computing.html
As for microwave ovens and other appliances, if updating software is not a normal part of use of the device, then it is not a computer. In that case, I think the user need not take cognizance of whether the device contains a processor and software, or is built some other way. However, if it has an "update firmware" button, that means installing different software is a normal part of use, so it is a computer.