If you delete the existing build folder and try to build project, it won't compile (see attached screenshot). To make it work, you will need to either 1) do Build -> Rebuild Project or 2) do Build -> Clean Project or 3) close the project in Qt Creator, delete %projectname%.qbs.user file and reimport the project. For me, as for new qbs user, it is very annoying because qmake works normally in this case.

For your convenience I'm attaching a simple project which has both .pro and .qbs files.