On what grounds should earth-contacting walls be included on top? That would be legally untenable in terms of contract law if they offer a complete residential building on this plot (which is specifically named and surveyed by them), according to their construction service description, and then afterward say earth-contacting walls were not part of the offer?!
You just gave yourself the answer. The construction service description specifies a certain type of wall under certain conditions. If these are not met --> additional costs. Since the construction service description is part of the contract and you signed the contract, hardly any judge will side with you. The condition of the plot is always the responsibility of the builder, not the construction company.
We do not know the company's construction service description, and my hobby is not to play detective (throwing a picture from the realtor into Google image search after it has already been extensively discussed here is something else). But you should take such points into account and check the construction service description carefully. And if you are unsure, clarify and have the construction service description changed (or clarified in an attachment). You have nothing to lose by doing so; in the worst case, you realize that your construction company does not want to cover the costs.