I tried and failed to find their patent application. I used Google Patents and the search queries [ "az" (pipe|tube) flow ] and [ (olivier|dopson) (pipe|tube) flow ], with filing date on or after Jan 1, 2010. It might have been filed before 2010, or it might not have been published yet.
I read the comments on the KS page just now. They say Osiris has ten gain levels, which it automatically chooses from to adapt to different pipes, and its detection threshold is quite low (dripping faucet, in their tests). They also say it can report usage in metric or American units. These suggest that they are maybe doing some calculation of flow rate.