There is a false assumption that manufacturers don't use Linux because of GPL. Most products don't use Linux because they don't need it, and they use something like FreeRTOS or another commerical RTOS. Plenty of companies use Linux in all sorts of home appliances, mostly TV products, but an increasing number of fridges from companies like Samsung do have Linux and they do release the code. What manufacturers don't do is release the applications that they run on the OS (despite the false assumption that they should).

The GPL requires that any changes (made to code used in released products) be released under the GPL. I'm unaware of any home appliance manufacturer that freely releases the OS of their products under the GPL, as they would have to if they were using Linux.

I'm sorry, that was a brainfart- I thought you meant the connector and the host device and not quite literally a random microwave and oven :)