My reading of the Nokia phone I linked was that it was a pretty minimal system, so no I wouldn't say
that phone was a smartphone. Ever used a system with Windows or macOS running in kiosk mode? Where it's really just an embedded system to let you look up train times or find a book in a library? That's not a general purpose computer anymore, even though it's running on the hardware and OS that would produce a general purpose computer. It has been constrained to the point that it is now a specialized feature computer.
I'd draw the same line between feature phones running a "smartphone" OS and actual smartphones. If you constrain the OS so that it can't run to its potential, at some point you cease to have a smartphone (even if technically it could be). Just like an iPhone isn't really a general purpose computer. It could be, but in practice it is not, because of the constraints placed on the system via iOS and the App Store.