Another aspect to this is that Android is Linux, but it is not GNU / Linux. This is true both in the literal sense of not using GNU coreutils or glibc, and also in the broader sense.
What I mean by the "broader" sense:
no X or Wayland
GTK or Qt support is something an application has to bring with them.
filesystem is substantially reorganized
users and system permissions setup substantially differently
To the application programmer Android / Linux looks like a completely different ball game.
Its like saying MAC is UNIX, technically yes but not really
macOS is UNIX-based. Period. In fact, “[s]tarting with Leopard, macOS has been certified as compatible with the Single UNIX Specification version 3 (SUSv3).” Source
As far as I can tell literally everything (except for windows) is UNIX, it isn’t exactly a high bar to pass, it’s like saying all drinks are water based which is technically true but I’m not going to give a kid a wine box when I run out of juice boxes.
I think that Linux is synonymous with open source and freedom and to some extent privacy.
android is synonymous with Google
I’m only guessing, but when people ask about a Linux tablet or phone, they actually want the former, rather than the later, even though android is a Linux derivative, as you rightly said.