Better rephrase that to indicate that they have no more control over you or the device than the non-replaceable firmware allows, this is definitely not the same as "no control". Non-replaceable firmware can ping home, it can be used to monitor users, it can download arbitrary code and run it, it can listen for some trigger to start messing with your data or downright destroy said data, it can start randomly rebooting systems or non-randomly restart all affected systems at a specific time - which may cause brown- or blackouts depending on the power draw and -supply, etcetera.