Yes, there's no straight rules. The author can bend the omegaverse concept however they want. The information you see in the first pages are made so that people reading omegaverse for the first time can have the basic idea of what it is, but it's not a set of rules. If you actually read the info in English, they say that it's all really up to the author what happens in the story.
Like everyone is trying to explain, there are no set rules for the omegaverse. Those rules are the basic rules for making this kind of stories, but any author is free to change or adjust them. If you read kashikomarimashita destiny for example, anyone can be pregnant, even an alpha can bear an omega child. I like to think that sex pistols is a precursor of the omegaverse, even if the setting is more complex.

Only omega male can give birth cuz they have womb , alpha males don't have womb so them marrying would not make any sense by omegaverse world rules