Agreed. Products/companies can be anticompetitive even if they are not a monopoly. Products can also contain their own submarket in which they achieved a monopoly through anticompetitive practices. For example ios on Iphones. That last bit is somewhat controversial but is the opinion of the current justice department of the executive branch of the government and seems to be the correct reading of the Kodak case[0]. In any event I don't think Prime video is anticompetitive except in as much as it can take losses for years to price others out of the market but that doesn't seem to be happening.

[0]https://en.wikipedia.org/wiki/Eastman_Kodak_Co._v._Image_Tec....