You aren't entitled to getting things the way you want them, it all comes down to that awesome fact (or sad, if you're one of those people who get triggered when business owners don't bake the cake for them) in the end. You having some issues with that is irrelevant - you can try going the Zoe Quinn route and start living in some alternative reality, but it won't change the actual reality. So, in short, there is absolutely nothing wrong about there being no online co-op. So, basically, the game description states that there is no online co-op, millions of people have bought this game and see no problem with that fact. The only one having issues understanding things is definitely you you keep speaking about some fact X being the objective truth based on your and your only opinion about that, completely ignoring the fact that the vast majority of people who bought this game do not hold this opinion.

Now my question is whether or not they were going to add it in later, which you have not answered. Co-op is core to the gameplay, and whether it was promised or said to have been included or not, the fact that online multiplayer is not a feature is absolutely terrible. Again, since you seem to have trouble understanding my point, the fact that it says on the steam store that the game does not have online multiplayer is irrelevant.

Online multiplayer cuphead