This is a dumb argument. Yes, my phone uses Linux. How many of the Android users actively come in contact with the underlying system?
Mainstream Linux means a big part of people actively choosing to install a Linux distribution or buy a computer or notebook with a real Linux distro pre installed (not that lightweight barebones distro they preinstall so they can sell it without Windows but with OS).
I use Gentoo, the family PC has OpenSUSE, only my wife’s laptop has Windows… Because guess what, she wants to use what she’s used to, what she knows.