Spoofing the User-agent string and then using Trident to render is not only possible, it's common in several "alternate browser" apps.
A more telling test is to run some browser benchmarks and compatibility tests. For example, IE10 (WP8.0) gets 100/100 on Acid3 (http://acid3.acidtests.org
) while Opera Mini only gets 97/100, and also slows down at different points in the test. There are other differences too, such as how the browser reacts to taps and zooming, that make it pretty clear it's not using a WebControl.