That's correct. Any functionality that can be updated and change how the system works at the whim of a third party (not the user) is essentially a backdoor.

What would you propose as the definition of a backdoor that does not include the above?

automatic updates without the user's consent are the very definition of a 'backdoor.' just because a new name for it "update" has been commercially normalized doesn't mean the original name has lost its meaning.

Alright, then everything is a backdoor. Snaps are backdoors. Windows is a backdoor. MacOS is a backdoor. Watches are backdoors. Every cell phone is a backdoor. Thermostats and headphones and book readers are backdoors. Every car currently sold new is a backdoor. 90% of every device we interact with is a backdoor. So why put that in the title? It means nothing.

Very close. 90% of the computerized devices we interact with have backdoors. But then you jump to "backdoors must not be a problem" instead of "we have a severe problem all around us."

Not everything is a backdoor, but Windows absolutely is and that's part of why people dislike Windows and voluntarily use the older versions. MacOS, and some cell phone OS's, at least let the user choose if/when to update in which case they're not backdoors.