Engines operate on a lot of fluid dynamics assumptions. Every time a valve is opened or closed there is a sonic boom as the pressure differential is so great that the gases moving around break the sound barrier, there's a shockwave from this that can be used to aid (or hinder) in airflow not only to the single cylinder but to the system as a whole. By running simulations and some experimentation an intake system is tuned to the engine for certain characteristics: sound, efficiency, performance etc. by varying the size of the resonant chamber which is what this mod does.
Pretty picture of a 2 stroke engine which relies on exhaust tuning to get more power and efficiency:
Now the real question is "is this too good to be true?"
My answer is "probably", if 20hp was this simple Toyota would have done it. Maybe the idle is rough, probably to get the most out of it the tune required (it is required for the gains they claim) isn't compliant with emissions. I'd gamble that there's a folder in Toyota's hard drives with at least a preliminary analysis of this intake design that was discarded for some reason or another.
If you want 20hp, don't care about passing emissions or an aftermarket reflash then it looks pretty trick to me.