They are, you don’t need 100% of a market to be considered a monopoly, just a big enough share that you’re considered to be the default option.

Valve are actually getting sued for having anti competitive provisions in their contracts preventing devs from selling their games for less on other stores.

I also doubt you wouldn’t complain about Valve if they made you want to drop the service, they don’t necessarily need to ban you for you to regret your decision to only buy from them, it could simply be a scandal about internal issues or them introducing features your disagree with to increase their profits (which they could do considering their position in the market).