Apple and Google have a duopoly on Mobile OS. If Apple uses Google's model for Siri, that means Apple and Google are using their duopoly in one market (mobile OS) to enforce a monopoly for Google in another (model for mobile personal assistant AI).

They are in a duopoly on the Mobile OS market, with no other significant player available. Google would be the sole integrated mobile AI, though there are competitors available if customers wanted to switch (customers for such products being the OS companies buying the AI services, not the end-users).

However I don't see the link, how they are "using their duopoly", and why "they" would be using it but only one of them benefits from it. Being a duopoly, or even a monopoly, is not against anti-trust law by itself.