Not making excuses for anyone. Physics are physics and if there is a strong signal you'll get interference. WiFi interference happens with A&K players, Oppo HA-2, Mojo, and almost every other thread I've read on a device with WiFi capabilities, or connections with smartphones.
Sure, perhaps FiiO could have shielded the WiFi better but then again some users get noise and others don't. I'm sure that the physical manufacturing can't be that different between units so the only thing left is different routers, router settings, different signal strengths etc, etc,. Also, as highlit in the Mojo thread, the cables can basically act as an atennae and pick up noise/EMI. It sucks but it happens.
My earlier remark was more pointed at the comments that another player without WiFi was a consideration. If going that route then why not simply turn off WiFi?