Defamation. What Roku is alleging is a direct attack on Google's reputation and if Google is telling the truth then it should be easy to verify in communications between Roku and Google.
There is a simple way for Google to disprove Roku: just support the device. If they don't do that, everything else is just a smoke screen operation.
They don't want to pay Roku money. It wouldn't be natural to demand someone clear their good name by paying money (i.e. 'support' the device) to the person who insulted them.
That's akin to blackmail.
Roku has a platform. If google wants to be on it, they have to be OK with giving them a cut. Why is that so difficult?