Seems that the problem is only present with the latest (or later) version(s) of XPUIPC.
Here a user was able to resolve it by reverting to the initial 2.0.0.0 release:
http://forums.x-pilot.com/forums/topic/14345-assigned-throttle-axes-dont-work/
Although the author's site seems to be down or under construction, there's an archived version available: https://web.archive.org/web/20171109165154/http://www.tosi-online.de/XPUIPC/XPUIPC.html