If not, then it is a de-facto locked-down device. It shouldn't take technical expertise to keep consumer electronics usable.
I'm not criticising your point; just nitpicking your use of “grandma” as a signifier for the kind of person who can't deal with how garbage modern computers are. It's stereotypical, and only makes sense if you know those stereotypes; that's perpetuating stereotypes, and it also makes your writing inaccessible for people not familiar with the way your culture sees old women.
Pretty much every technology invented by humanity gets better with knowledge and training. That doesn't make those technologies "locked down".
A more useful definition would be that a device is locked down when the manufacturer has taken measure to halt users from loading their own software, including their own OS.
There might be reason to claim Chromebooks aren't locked down, go flip developer mode on and off you go. As for the iPad case, Apple most definitely does not want you to run your own OS. Older iOS devices often have cracks and ports of Android to them, but that takes considerable effort to defeat Apple's locking down of the device.
The comment I replied to suggests "my grandma can't figure out how to install Firefox on a Chromebook, so that means the device is locked down". I disagree with that.
There is no magic world in which complicated technology remains forever usable by the unskilled.