I am glad you are so sure. But I think you are wrong. If you look at the spec sheet for the mount (took a bit of finding 'cause the OP's link doesn't work), it seems to need to screw into the star-nut in a threadless steerer. The stem adapter linked does not have a place to screw in a bolt, and the existing bolt in the adapter needs to be torqued down tight to hold the wedge in place and keep the bars from falling off.

Edit:

There was a mention her a few weeks ago about a steerer tube extender designed to work with a too-short steerer in a threadless headset. I don't know who made it, but that might have the required place to put a bolt that would preload the headset and then (after clamping the stem bolts down to hold the headset together), remove to make space for the holder for the e-phone.