There is no “monopoly” on either content distribution or creation. Amazon and Apple are both trillion dollar companies that have streaming services.
Then there is Disney, Comcast (Peacock), Paramount, STARZ (standalone company), and AMC
Technically, you're right. I feel like there needs to be new terms to describe though the staleness of the industry. "Oligopoly" just doesn't have the same ring to it.
IMO I think we are going to see Paramount, STARZ and AMC bought up soon. I don't think they can compete with Disney, Comcast or Netflix in size.
The legal definition of monopoly in some jurisdictions means anyone with a large enough of a market share able to influence pricing, etc in a market. A market share as low as 25% can be called a monopoly. Does HBO+Netflix have a 25% share of the streaming market? I've no idea, but possibly.