So, if I have a keyboard on my tablet that is running Android?
I think the point here is that "desktop" as a location has no real meaning - it's the use case that defines it rely. By desktop system one really means a day-to-day used system that is used for word-processing, web browsing and other standard domestic/work uses (photo editing, spreadsheets, games and such). That's the stuff a desktop system has always done and now you can do that on a phone, phablet, tablet, laptop, desktop or whatever. The hardware form is just about convenience it [no longer] describes a logical difference or significant processing ability [eg my phone has about as much processing power as my desktop (ignoring the GPU), some phones definitely have more]
If you're using your phablet paired with a keyboard for the same uses as others use their [traditionally defined] desktop for then what use is the distinction?
Android can be used on the desktop, but IMO it's not tailored for that and so shouldn't be described as a desktop OS.
The whole "year of Linux on the desktop" thing is about market-share as much as anything; as other have said many people have been using it as a desktop OS for plenty of years.