That's a nice calculator, but it's not a simulator. I haven't downloaded it, but it looks like all of the tone circuits are passive. It also allows you to change the source impedance of the signal source, which will change the results of the EQ curves. That it calculates the affect of different source impedance's with the tone stack shows that outside influences do make a difference... that was never in question. The only way to see if calculated and simulated are not the same as reality is to physically take actual measurements.
All electronic circuits are representations of mathematical functions. If there are discrepancies between the calculated/simulated with the actual physical measurements, then something was not included or overlooked with those calculations and simulations.
-Frank
Simulator or calculator is neither here nor there really - the OP asked for better understanding of how tilt EQ works - this tool provides modelling and illustration of a number of passive tone stacks - nothing more, nothing less.
