He showed it on the x3 because the build would cause a bootloop on the 950. That's a sign of things to come (I know, MS could always add support for the 950 in future builds)
Most likely the x3 was the reference hardware for the pre-alpha. HP work pretty closely with ms these days. I doubt this current pre-alpha would run on anything but the x3 (or its refresh)
It's really going to be up to MSFT what devices they will offer support to, but it would seem a bit counter-productive to only have it run on a device that almost nobody owns. The 950/950XL and 4s are all more common than the x3 in the wild. And up until recently the 950 has been used as reference hardware (which is why the 4s is sometimes a bit buggy).
I think the big FCU for mobile will be like 7.8 in a way, possibly the last big update for existing hardware (it should include cshell, timeline, Cortana skills, nightlite - a real big number - no point in that for one rarely used phone). If that's true it would make sense to include all the big four in it. Help keep the fire burning a little for the transition to whatevers next (64 bit and 835 most likely, if MSFTS rumoured three mobile devices and HPs refresh can draw in some more OEMs and some consumer interest).
The fact that this is designed to run on 32 bit at all is telling. If they were simply 'dumping their existing consumers' (which would be a bit silly), they wouldn't bother writing it for 32 bit. That they have suggests they'll extend it to 950/xl at minimum.
Just seems logical to me, but I guess no one knows for sure.
Although to be fair, I'm not super fussed. I mainly use continuum for streaming not work. I've got a desktop, tablet and mobile. My tablet has a little BT keyboard and mouse. Cshell is great for the OS vision, but I wouldn't really miss it. I'm probably more exited about large tiles than continuum multi-window, lol.
In fact as far as cshell goes, I'm more interested in the console/desktop meeting point and tablets - if there will be any mid point scaling there. I very much like how everythings coming together though...