No, cellular was off on both phones. No case.
But I redid the test just this morning, and got different results (I think the etxernal Internet might have been slower when I did the first test, though that doesn't explain the atrocious performance of the Icon). The 521 is still faster, but not as much. The reason it doesn't win is frankly due to less capable hardware, in every test the 521 download freezes and resumes in the middle of it, as if it has trouble keeping up with the download. The uploads are twice as fast on the 521 compared to the Icon though, which is probably a better measure of the real difference in WiFi capability.
Screenshots (Icon on the left, Lumia 521 on the right):
View attachment 66890
I also tested both phones in the parking lot of a nearby McDonald's... the Icon could not find any network, where the 521 picked up several.
View attachment 66891
And finally, took screenshots of the WiFi settings at home, where the 521 has a stronger signal and picks up additional networks of neighbors.
View attachment 66892
Clearly, the Icon WiFi capability using the same OS (8.1) is much worse than on my Lumia 521. That's just not right...