No, it's because it's compatible with the LGPL. LGPL != GPL. Your conclusion is correct (you can license your app under the MIT license), but your reasoning is wrong (not because it's compatible with the GPL). You can use Qt under the GPL, in which case you have to license your code as GPL, or you can use Qt under the LGPL, in which case you can license your code however you want (or keep it closed source), so long as you properly use/link to Qt (namely, dynamically link to it).
So if I use QT to develop my application, then the overall application could be, say, MIT, because it is compatible with GPL?
You can dual-license a library and allow people to use it under two different licences (for example, GPL and LGPL), whichever they prefer. You can triple-license, or N-license (where N is any number) it, and let users choose which license to use (or pay for the option to use a particular license).