Frequency of transit in planar and non-planar planetary systems
This is unfortunately not true.
If you already look at our solar system, even this is not co-planar.
The few degrees inclination that the planets have to each other would already be enough to make some of them undetectable, when others are detectable by transits. 

The answer to your question therefore is not so much about the transit probability, but more about what the mutual inclinations are of the planets. Those can be stirred up by close-planet encounters or resonances after the gas disc dissipates in which the planets form.