That is not a realistic definition, to me. It is a question of how the term has become misused. Basically "App" is an abbreviation for Application.
"app is single device (by type) so Windows phone only, windows 10 only or Ipad only = app"
OK, but I have currently, by default only, 54 "Apps" running on my Desktop?
This may be Microsoft misusing the term, or..?
Expanding your line, the so-called App is supposed to be something which only runs online or on a mobile device. Huh??
Like most of us, for many years I used the term App, as an abbreviation for Application. I don't know how it became more associated with mobile devices, but, delving through, it seems that when the Ipad came out, the abbreviation became a word of its own.